Greenbrook TMS (GBNHF) 424B5Prospectus supplement for primary offering

This document consists of two parts. The first part is this prospectus supplement, which describes the specific terms of this offering and adds to and supplements information contained in the accompanying base prospectus and the documents incorporated by reference therein. The second part is the accompanying base prospectus, which gives more general information, some of which may not apply to this offering. This prospectus supplement forms a part of the accompanying base prospectus solely for the purpose of this offering. This prospectus supplement and the accompanying base prospectus do not contain all of the information set forth in the Registration Statement (as defined below), certain items of which are contained in the exhibits to the Registration Statement as permitted or required by the rules and regulations of the SEC. Items of information omitted from this prospectus supplement and the accompanying base prospectus, but are contained in the Registration Statement, are available on EDGAR at www.sec.gov.

A prospective purchaser of the Offered Shares should read this entire prospectus supplement and the accompanying base prospectus, including the documents incorporated herein and therein by reference, and consult its own professional advisors to assess the income tax, legal, risks and other aspects of its investment in the Offered Shares. A prospective purchaser of the Offered Shares should rely only on the information contained and incorporated by reference in this prospectus supplement and the accompanying base prospectus. The Company and Alumni Capital has not authorized anyone to provide the prospective purchaser of the Offered Shares with additional or different information. The information contained or incorporated by reference in this prospectus supplement and the accompanying base prospectus is accurate only as of their respective dates, regardless of the time of delivery of this prospectus supplement and the accompanying base prospectus or any offer or sale of the Offered Shares. The Company’s business, financial condition, results of operations and prospects may have changed since those dates.

The Offered Shares may be sold only in those jurisdictions where offers and sales are permitted. This prospectus supplement and the accompanying base prospectus is not an offer to sell or a solicitation of an offer to buy the Offered Shares in any jurisdiction where it is unlawful.

If the information varies between this prospectus supplement and the accompanying base prospectus, the information in this prospectus supplement supersedes the information in the accompanying base prospectus.

Unless otherwise noted or the context otherwise requires, the “Company”, “Greenbrook”, “we”, “us” or “our” refer to Greenbrook TMS Inc. together with its subsidiaries.

Certain calculations included in tables and other figures in this prospectus supplement and the accompanying base prospectus, and the documents incorporated by reference herein and therein, have been rounded for clarity of presentation.

An investment in the Offered Shares is subject to a number of risks, including those set forth herein and in the accompanying base prospectus, as well as in the Annual Report and the Q1 2023 Interim Report, each of which are incorporated by reference herein. Prospective investors should carefully consider these risks, in addition to information contained in this prospectus supplement, the accompanying base prospectus and the information incorporated by reference herein and therein, before purchasing Offered Shares.

Based on recurring losses from operations and negative cash flows from operations for the three months ended March 31, 2023 as well as current cash and liquidity projections, we have concluded that there is substantial doubt about our ability to continue as a going concern. Furthermore, as discussed in Note 2(a) to our financial statements for the year ended December 31, 2022, included in the Annual Report, we have generated operating losses since inception, and we had negative cash flow from operating activities for the year ended December 31, 2022, which together raises substantial doubt about our ability to continue as a going concern. In addition, there have been incidents where payments to vendors have been delayed until we were able to secure the requisite financing necessary to make such payments. In June 2023, we amended our Credit Facility to temporarily reduce our Minimum Liquidity Covenant from $3,000,000 to $1,000,000 and on July 3, 2023 it was further reduced to $300,000 through July 14, 2023 (see “Prospectus Supplement Summary — Recent Developments — Amendment to Credit Facility”). We do not expect to receive the proceeds of this offering by July 14, 2023 and will, therefore, require a further amendment to the Credit Facility in order to temporarily reduce our Minimum Liquidity Covenant. We can provide no assurances that we will be able to successfully obtain an additional waiver to our Credit Facility on terms acceptable to the Company, or at all. Before giving effect to this offering, as of July 12, 2023, we had cash on hand of approximately $0.6 million.

To the extent that we continue to have negative cash flow in future periods, we will need to allocate a portion of the net proceeds received from this offering and/or additional financings to fund such negative cash flow. There can be no assurance that additional capital or other types of financing will be available when needed or that these financings will be on terms at least as favorable to the Company as this offering, or available to the Company at all. In addition, we can provide no assurances that we will be able to satisfy payment obligations on our outstanding indebtedness (including, without limitation, the Credit Facility, the Neuronetics Note and the Klein Note).

You should not rely on our most recent consolidated balance sheet as an indication of the amount of proceeds that would be available for us to satisfy claims of our creditors (including, but not limited to, Madryn, Neuronetics, and Benjamin Klein), and potentially be available for distribution to shareholders, in the event of liquidation.

The net proceeds of the offering are only sufficient to enable to the Company to satisfy its cash obligations and comply with the Minimum Liquidity Covenant in the short term. The Company enacted its proposed Restructuring Plan in an effort to achieve certain cost savings; however, if the Company does not consummate this offering and/or obtain additional financing, irrespective of potential cost savings from the Restructuring Plan, the Company may not have the financial resources to satisfy its cash obligations and maintain compliance with the Minimum Liquidity Covenant in the future. Our obligations under the Credit Facility are secured by first priority liens on substantially all assets of the Company and certain of its subsidiaries, subject to customary exceptions. In the event that Madryn delivers a notice of acceleration to the Company in such case, we expect that Madryn would foreclose on the collateral and accelerate the indebtedness under our Credit Facility, following which the Company will likely be required to file for bankruptcy protection, and our

Trading in our Common Shares is highly speculative and poses substantial risks to investors. There is no guarantee that an investment in the Offered Shares will earn any positive return in the short or long term, and we have encountered substantial cash shortages that have required restructuring of our outstanding balance pursuant to the Neuronetics Agreement, amendments and waivers under our Credit Facility, as well as the extension of additional loans from Madryn and additional financing from significant shareholders (including Greybrook Health) and management, and led to us adopting the Restructuring Plan. A purchase of Offered Shares involves a high degree of risk and should be undertaken only by investors whose financial resources are sufficient to enable them to assume such risks, who have no need for immediate liquidity in their investment and who have the financial capacity to absorb a loss of some or all of their investment. Trading prices for our Common Shares may bear little or no relationship to the actual recovery, if any, by holders of our securities in any bankruptcy proceeding and our shareholders will likely not receive any recovery at all in a bankruptcy scenario. Our operations and ability to develop and execute our business plan, our financial condition, our liquidity and our continuation as a going concern, are subject to risks and uncertainties. These risks include, but are not limited to, the following:

Following completion of this offering (assuming issuance of all of the Purchase Shares at the Assumed Offering Price) there will be 49,328,648 Common Shares issued and outstanding. If all of the Company’s options that were issued and outstanding as of July 11, 2023, including options that are not yet exercisable, were to be exercised, the Company would be required to issue up to an additional 1,689,000 Common Shares, or approximately 3.4% of the issued and outstanding Common Shares following completion of this offering (assuming issuance of all of the Purchase Shares at the Assumed Offering Price) on a non-diluted basis. In addition, there are also 187,177 warrants of the Company issued and outstanding that are exercisable into Common Shares on a one-for-one basis at a weighted average exercise price of $3.65. If all of these warrants were to be exercised, the Company would be required to issue up to an additional 187,177 Common Shares, or approximately 0.4% of the issued and outstanding Common Shares following completion of this offering (assuming issuance of all of the Purchase Shares at the Assumed Offering Price) on a non-diluted basis. In addition, the Company has outstanding under the Madryn Conversion Instruments that are convertible at Madryn’s option into up to 2,918,659 Common Shares at the Madryn Conversion Price, and the Greybrook Conversion Instrument that is convertible into $1.0 million of Common Shares at the Greybrook Conversion Price (up to 2,708,162 Common Shares). If the entirety of the Madryn Conversion Instruments were to be exercised, the Company would be required to issue an additional 2,918,659 Common Shares, or approximately 5.9% of the issued and outstanding Common Shares following completion of this offering (assuming issuance of all of the Purchase Shares at the Assumed Offering Price) on a non-diluted basis. Finally, if the Company defaults in respect of its obligations under the Neuronetics Note, Greenbrook will be required to issue the Neuronetics Warrants in an amount equal to (i) 200% of the unpaid amount of any delinquent amount or

Sales of a substantial number of the Common Shares in the public market could occur at any time, including by any of our major shareholders, directors or officers. Such future sales may include sales of Common Shares by Mr. Klein, whose one-year lock-up expires on July 14, 2023. These sales, or the market perception that the holders of a large number of Common Shares intend to sell their Common Shares, could significantly reduce the market price of the Common Shares. We cannot predict the effect, if any, that future public sales of Common Shares or the availability of these securities for sale under this prospectus supplement will have on the market price of the Common Shares. If the market price of the Common Shares was to drop as a result, this might impede our ability to raise additional capital and might cause remaining shareholders to lose all or part of their investment.

The Common Shares issuable upon the exercise of outstanding options, warrants and conversion instruments will, to the extent permitted by any applicable vesting requirements, lock-up restrictions, and restrictions under applicable securities laws, also become eligible for sale in the public market. Further, we have previously filed a resale prospectus for resales of up to 2,353,347 Common Shares that were issued to investors in a private placement in June 2021. We are also obligated to register the resale of (1) up to 11,634,660 Common Shares that were issued to the Success TMS sellers (including Mr. Klein) in July 2022, which includes 2,908,665 Common Shares that have been held back and deposited with an escrow agent, to be released to Mr. Klein or the Company, as applicable, upon satisfaction of customary working capital and certain other adjustments, including to satisfy any indemnity claims against the Success TMS sellers; (2) up to 2,918,659 Common Shares issuable upon conversion of the Madryn Conversion Instruments; and (3) up to 11,363,635 Common Shares that were issued in connection with the non-brokered private placement completed in March 2023, in each case, once the conditions for satisfying such registration rights are satisfied. We cannot predict the size of future sales of any such Common Shares or the effect, if any, that future sales of any such Common Shares will have on the market price of the Common Shares.

If you invest in our Offered Shares in this offering, your ownership interest will be immediately diluted to the extent of the difference between the applicable price (depending on the Purchase Order Option elected by the Company) of the Offered Shares and the as adjusted net tangible book value per share of our Common Shares after this offering, assuming issuance of all of the Purchase Shares at the Assumed Offering Price.

Our net tangible book value as of March 31, 2023, was approximately ($104.5 million), or ($2.56) per Common Share. Our net tangible book value is the amount of our total tangible assets less our total liabilities. Net tangible book value per share is our net tangible book value divided by the number of Common Shares outstanding as of March 31, 2023.

Without taking into account any other changes in our net tangible book value after March 31, 2023, other than to give effect to the sale and issuance of an aggregate of $4,458,156 in Offered Shares in this offering, our as adjusted net tangible book value as of March 31, 2023 would have been approximately ($100.2) million, or approximately ($2.03) per share (based on the Assumed Offering Price of $0.52 per Common Share). This amount represents an immediate increase in as adjusted net tangible book value of $0.53 per share to our existing shareholders and an immediate dilution in as adjusted net tangible book value of approximately ($1.51) per share to new investors purchasing Common Shares in this offering, assuming issuance of all of the Purchase Shares at the Assumed Offering Price. We determine dilution per share to new investors by subtracting the as adjusted net tangible book value per share after this offering from the amount of cash that a new investor paid for a Common Share in this offering. The following table illustrates this dilution:

The foregoing table and discussion is based on 40,800,180 Common Shares outstanding as of July 11, 2023, and excludes (as of July 11, 2023):

This prospectus provides you with a general description of the Securities that we may sell under this prospectus. Each time we or the selling shareholders sell Securities, we will also provide a prospectus supplement that may include, where applicable, specific information about the terms of that offering. If there is any inconsistency between the information in this prospectus and any applicable prospectus supplement, you should rely on the information in the prospectus supplement. Where required by statute, regulation or policy, and where Securities are offered in currencies other than U.S. dollars, appropriate disclosure of foreign exchange rates applicable to those Securities will be included in the prospectus supplement describing those Securities.

We may also prepare free writing prospectuses to describe the terms of particular sales of Securities, which terms may vary from those described in any prospectus supplement. You therefore should carefully review any free writing prospectus available in connection with your review of this prospectus and any applicable prospectus supplement.

Please carefully read both this prospectus and any prospectus supplement, together with the documents incorporated by reference into this prospectus and any prospectus supplement, and the additional information described below under “Where You Can Find Additional Information”. This prospectus contains summaries of certain provisions contained in some of the documents described in this prospectus, but reference is made to the actual documents for complete information. All the summaries are qualified in their entirety by the actual documents. Copies of certain documents referred to in this prospectus have been filed, will be filed or will be incorporated by reference as exhibits to the Registration Statement, and you may obtain copies of those documents as described below under “Where You Can Find Additional Information”.

You should rely only on the information contained in or incorporated by reference into this prospectus and any prospectus supplement or free writing prospectus. We and the selling shareholders have not authorized anyone to provide you with any different or additional information other than that contained in or incorporated by reference into this prospectus or in any prospectus supplement or free writing prospectus. We and the selling shareholders take no responsibility for, and can provide no assurance as to the reliability of, any information that others may provide. The distribution or possession of this prospectus in or from certain jurisdictions may be restricted by law. This prospectus is not an offer to sell any Securities and is not soliciting an offer to buy Securities in any jurisdiction where the offer or sale is not permitted or where the person making the offer or sale is not qualified to do so or to any person to whom it is not permitted to make such offer or sale. The information contained in this prospectus is accurate only as of the date of this prospectus and any information incorporated by reference into this prospectus is accurate only as of the date referred to in the applicable document incorporated by reference, regardless of the time of delivery of this prospectus or

This forward-looking information and other forward-looking information are based on the Company’s opinions, estimates and assumptions in light of its experience and perception of historical trends, current conditions and expected future developments, as well as other factors that the Company currently believes are appropriate and reasonable in the circumstances. Despite a careful process to prepare and review the forward-looking information, there can be no assurance that the underlying opinions, estimates and assumptions will prove to be correct.

Various assumptions or factors are typically applied in drawing conclusions or making the forecasts or projections set out in forward-looking information. Those assumptions and factors are based on information currently available to the Company, including information obtained from third party industry analysts and other third-party sources. In some instances, material assumptions and factors are presented or discussed elsewhere in this prospectus or the documents incorporated by reference herein in connection with the statements or disclosure containing the forward-looking information. Readers are cautioned that the following list of material factors and assumptions is not exhaustive. The factors and assumptions include, but are not limited to: no unforeseen changes in the legislative and operating framework for the Company’s business; no unforeseen changes in the prices for the Company’s services in markets where prices are regulated; no unforeseen changes in the reimbursement rates of commercial, Medicare and other non-Medicare government insurance plans; no unforeseen changes in the regulatory environment for the Company’s services; a stable competitive environment; and no significant event occurring outside the ordinary course of business.

The Company will not receive any proceeds from any sale of Securities by the selling shareholders.

The applicable prospectus supplement will include details of the warrant indentures, if any, governing the Warrants being offered. The specific terms of the Warrants, and the extent to which the general terms described in this section apply to those Warrants, will be set out in the applicable prospectus supplement. The prospectus supplement relating to any Warrants the Company offers will describe the Warrants and the specific terms relating to the offering. The description will include, where applicable:

Warrant certificates will be exchangeable for new Warrant certificates of different denominations at the office indicated in the prospectus supplement. Prior to the exercise of their Warrants, holders of Warrants will not have any of the rights of holders of the securities subject to the Warrants. The Company may amend the warrant indenture(s) and the Warrants, without the consent of the holders of the Warrants, to cure any ambiguity, to cure, correct or supplement any defective or inconsistent provision, or in any other manner that will not prejudice the rights of the holders of outstanding Warrants, as a group.

The Company may issue Subscription Receipts separately or together with Common Shares, Preferred Shares or Warrants, as the case may be. The Subscription Receipts will be issued under a subscription receipt agreement. This section describes the general terms that will apply to any Subscription Receipts that may be offered by the Company pursuant to this Prospectus.

The applicable prospectus supplement will include details of the subscription receipt agreement covering the Subscription Receipts being offered. A copy of each subscription receipt agreement relating to any offering of Subscription Receipts will be filed by the Company with the SEC after it has been entered into by the Company, and will be incorporated by reference as an exhibit to the Registration Statement. The specific

Up to 2,353,347 Common Shares may be sold under this prospectus by way of secondary offering by or for the account of selling shareholders identified below to whom we issued and sold such Common Shares in a private placement that was completed on June 14, 2021, as required by the registration rights agreement entered into connection with such transaction. For more information, see Item 5.B, “Operating and Financial Review and Prospects — Management’s Discussion and Analysis of Financial Condition and Results of Operations — Liquidity and Capital Resources — Related Party Transactions — Investor Rights Agreement; Registration Rights Agreement” in the Annual Report, which is incorporated by reference herein. Except as described herein and the Annual Report, which is incorporated herein by reference, and in the table and accompanying footnotes below, the selling shareholders have not had any material relationship with us or any associate or affiliate of a principal holder of our voting securities within the past three years. In addition, to our knowledge, none of the selling shareholders is a broker-dealer, nor at the time of the acquisition of our Common Shares did any of the selling shareholders have direct or indirect agreements or understandings with any third-party person to distribute any of our Common Shares.

The table below identifies the selling shareholders and provides other information regarding the beneficial ownership of the Common Shares by each of the selling shareholders, which information was provided to us by, or on behalf of, the applicable selling shareholder. Because the selling shareholders may sell, transfer or otherwise dispose of all, some or none of the Common Shares offered by this prospectus, we cannot determine the number of such Common Shares that will be sold, transferred or otherwise disposed of by the applicable selling shareholder or the amount or percentage of Common Shares that will be held by the selling shareholders upon termination of this offering. See “Plan of Distribution” for more information. For purposes of the table below, we assume that the selling shareholders will sell all of their Common Shares offered by this prospectus.

In the table below, the percentage of Common Shares beneficially owned is based on 17,801,885 Common Shares issued and outstanding as of March 31, 2022, determined in accordance with Rule 13d-3 under the Exchange Act. Under such rule, beneficial ownership includes any Common Shares over which any of the selling shareholders has sole or shared voting power or investment power and also any Common Shares that any of the selling shareholders has the right to acquire within 60 days of such date through the exercise of any options, warrants or other rights. Except as otherwise indicated, we believe that each selling shareholder has sole voting and investment power with respect to all Common Shares shown as beneficially owned by it. The beneficial ownership information presented in this table is not necessarily indicative of beneficial ownership for any other purpose. When we refer to the “selling shareholders” in this prospectus, we mean the selling shareholders listed in the table below, as well as each such selling shareholder’s pledgees, donees, assignees, transferees and other successors-in-interest and others who may hold any of such selling shareholder’s interest received after the date of this prospectus from such selling shareholder as a gift, pledge, partnership distribution or other non-sale related transfer.

The Company and the selling shareholders may from time to time offer for sale up to an aggregate of US$150,000,000 (or the equivalent in other currencies based on the applicable exchange rate at the time of the offering) in Securities hereunder, including resales of up to 2,353,347 Common Shares as described in “Selling Shareholders”. Accordingly, the aggregate amount of Securities that we may offer in primary offerings under this prospectus is limited to US$142,516,356.54, after subtracting the registered amount of Common Shares that may be resold by the selling shareholders.

The Company and the selling shareholders may offer and sell the Securities to or through underwriters or dealers purchasing as principals, and may also sell directly to one or more purchasers or through agents or pursuant to applicable statutory exemptions. The prospectus supplement relating to a particular offering of Securities will identify each underwriter, dealer or agent, as the case may be, engaged by the Company or any selling shareholder in connection with the offering and sale of the Securities, and will set forth the terms of the offering of such Securities, including, to the extent applicable, any fees, discounts or any other compensation payable to underwriters, dealers or agents in connection with the offering, the method of distribution of the Securities, the identity of the selling shareholders, if any, the initial issue price, the proceeds that the Company or the selling shareholder, as applicable, will receive and any other material terms of the plan of distribution. Any initial offering price and discounts, concessions or commissions allowed or re-allowed or paid to dealers may be changed from time to time.

The Securities may be sold from time to time in one or more transactions at a fixed price or prices or at prices which may be changed or at market prices prevailing at the time of sale, at prices related to such prevailing prices or at negotiated prices, including sales in transactions that are deemed to be “at-the-market” offerings, including sales made directly on the TSX, NASDAQ or other existing trading markets for the Common Shares. The price at which the Securities will be offered and sold may vary from purchaser to purchaser and during the period of distribution.

In connection with the sale of the Securities, underwriters, dealers or agents may receive compensation from the Company, any selling shareholder or from other parties, including in the form of underwriters’, dealers’ or agents’ fees, commissions or concessions. Underwriters, dealers and agents that participate in the distribution of the Securities may be deemed to be underwriters for the purposes of applicable Canadian and/or U.S. securities laws and any such compensation received by them from the Company or any selling shareholder and any profit on the resale of the Securities by them may be deemed to be underwriting commissions.

In connection with any offering of Securities, except as otherwise set out in a prospectus supplement relating to a particular offering of Securities and other than in relation to an “at-the-market” offering, the underwriters, dealers or agents, as the case may be, may over-allot or effect transactions intended to fix, stabilize, maintain or otherwise affect the market price of the Securities at a level other than those which otherwise might prevail on the open market. Such transactions may be commenced, interrupted or discontinued at any time.

Underwriters, dealers or agents who participate in the distribution of the Securities may be entitled, under agreements to be entered into with the Company and/or any selling shareholder, to indemnification by the Company and/or any selling shareholder against certain liabilities, including liabilities under Canadian and/or U.S. securities laws, or to contribution with respect to payments which such underwriters, dealers or agents may be required to make in respect thereof. Such underwriters, dealers and agents may be customers of, engage in transactions with, or perform services for, the Company in the ordinary course of business.

This prospectus constitutes a public offering of these Securities only in those jurisdictions where they may be lawfully offered for sale and therein only by persons permitted to sell such Securities.