Affiliated Managers (MGR) 8-KEntry into a Material Definitive Agreement

Exhibit 5.1

Affiliated Managers Group, Inc.

777 South Flagler Drive

West Palm Beach, Florida 33401

Re:     Affiliated Managers Group, Inc. – Junior Subordinated Notes Offering

Ladies and Gentlemen:

We have acted as special United States counsel to Affiliated Managers Group, Inc., a Delaware corporation (the “Company”), in connection with the public offering of $200,000,000 aggregate principal amount of the Company’s 4.200% Junior Subordinated Notes due 2061 (the “Securities”) to be issued under the Indenture, dated as of March 27, 2019 (the “Base Indenture”), as amended and supplemented by the Third Supplemental Indenture, dated as of July 13, 2021 (the “Supplemental Indenture” and, together with the Base Indenture, the “Indenture”), between the Company and U.S. Bank National Association, as trustee (the “Trustee”).

This opinion is being furnished in accordance with the requirements of Item 601(b)(5) of Regulation S-K under the Securities Act of 1933, as amended (the “Securities Act”).

In rendering the opinions stated herein, we have examined and relied upon the following:

(a)    the registration statement on Form S-3ASR (File No. 333-230423) of the Company relating to debt securities and other securities of the Company filed on March 21, 2019 with the Securities and Exchange Commission (the “Commission”) under the Securities Act allowing for delayed offerings pursuant to Rule 415 of the General Rules and Regulations under the Securities Act (the “Rules and Regulations”), including information deemed to be a part of the registration statement pursuant to Rule 430B of the Rules and Regulations (such registration statement being hereinafter referred to as the “Registration Statement”);

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(b)    the prospectus, dated March 21, 2019 (the “Base Prospectus”), which forms a part of and is included in the Registration Statement;

(c)    the preliminary prospectus supplement, dated July 8, 2021 (together with the Base Prospectus, the “Preliminary Prospectus”), relating to the offering of the Securities, in the form filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations;

(d)    the prospectus supplement, dated July 8, 2021 (together with the Base Prospectus, the “Prospectus”), relating to the offering of the Securities, in the form filed with the Commission pursuant to Rule 424(b) of the Rules and Regulations;

(e)    an executed copy of the Underwriting Agreement, dated June 8, 2021 (the “Underwriting Agreement”), among the Company and Wells Fargo Securities, LLC, BofA Securities, Inc., Morgan Stanley & Co. LLC and RBC Capital Markets, LLC, as representatives of the several Underwriters named therein (the “Underwriters”), relating to the sale by the Company to the Underwriters of the Securities;

(f)    an executed copy of the Base Indenture;

(g)    an executed copy of the Supplemental Indenture;

(h)    the global certificate evidencing the Securities, executed by the Company and registered in the name of Cede & Co. (the “Note Certificate”), delivered by the Company to the Trustee for authentication and delivery;

(i)    an executed copy of a certificate of David M. Billings, General Counsel and Secretary of the Company, dated the date hereof (the “Secretary’s Certificate”);

(j)    a copy of the Company’s Amended and Restated Certificate of Incorporation, certified by the Secretary of State of the State of Delaware as of June 23, 2021 and certified pursuant to the Secretary’s Certificate;

(k)    a copy of the Company’s Amended and Restated By-laws, as amended and in effect as of the date hereof, certified pursuant to the Secretary’s Certificate;

(l)    a copy of the Company’s Amended and Restated By-laws, in effect on April 20, 2021, certified pursuant to the Secretary’s Certificate;

(m)    a copy of the Company’s Amended and Restated By-laws, as in effect on December 27, 2018, certified pursuant to the Secretary’s Certificate; and

(n)    a copy of certain resolutions of the Board of Directors of the Company, adopted on December 27, 2018 and April 20, 2021, and certain resolutions of the Pricing Committee thereof, adopted on July 8, 2021, certified pursuant to the Secretary’s Certificate.

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We have also examined originals or copies, certified or otherwise identified to our satisfaction, of such records of the Company and such agreements, certificates and receipts of public officials, certificates of officers or other representatives of the Company and others, and such other documents as we have deemed necessary or appropriate as a basis for the opinions stated below.

In our examination, we have assumed the genuineness of all signatures, including electronic signatures, the legal capacity and competency of all natural persons, the authenticity of all documents submitted to us as originals, the conformity to original documents of all documents submitted to us as facsimile, electronic, certified or photocopied copies, and the authenticity of the originals of such copies. As to any facts relevant to the opinions stated herein that we did not independently establish or verify, we have relied upon statements and representations of officers and other representatives of the Company and others and of public officials, including those in the Secretary’s Certificate and the factual representations and warranties contained in the Transaction Documents (as defined below).

We do not express any opinion with respect to the laws of any jurisdiction other than (i) the laws of the State of New York and (ii) the General Corporation Law of the State of Delaware (the “DGCL”) (all of the foregoing being referred to as “Opined on Law”).

As used herein, “Transaction Documents” means the Underwriting Agreement, the Indenture and the Note Certificate.

Based upon the foregoing and subject to the qualifications and assumptions stated herein, we are of the opinion that the Note Certificate has been duly authorized by all requisite corporate action on the part of the Company and duly executed by the Company under the DGCL, and when duly authenticated by the Trustee and issued and delivered by the Company against payment therefor in accordance with the terms of the Underwriting Agreement and the Indenture, the Note Certificate will constitute a valid and binding obligation of the Company, enforceable against the Company in accordance with its terms under the laws of the State of New York.

The opinions stated herein are subject to the following qualifications:

(a)    we do not express any opinion with respect to the effect on the opinions stated herein of any bankruptcy, insolvency, reorganization, moratorium, fraudulent transfer, preference and other similar laws or governmental orders affecting creditors’ rights generally, and the opinions stated herein are limited by such laws and orders and by general principles of equity (regardless of whether enforcement is sought in equity or at law);

(b)    we do not express any opinion with respect to any law, rule or regulation that is applicable to any party to any of the Transaction Documents or the transactions contemplated thereby solely because such law, rule or regulation is part of a regulatory regime applicable to any such party or any of its affiliates as a result of the specific assets or business operations of such party or such affiliates;

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(c)    except to the extent expressly stated in the opinions contained herein, we have assumed that each of the Transaction Documents constitutes the valid and binding obligation of each party to such Transaction Document, enforceable against such party in accordance with its terms;

(d)    we do not express any opinion with respect to the enforceability of any provision contained in any Transaction Document relating to any indemnification, contribution, non-reliance, exculpation, release, limitation or exclusion of remedies, waiver or other provisions having similar effect that may be contrary to public policy or violative of federal or state securities laws, rules or regulations, or to the extent any such provision purports to, or has the effect of, waiving or altering any statute of limitations; and

(e)    to the extent that any opinion relates to the enforceability of the choice of New York law and choice of New York forum provisions contained in any Transaction Document, the opinions stated herein are subject to the qualification that such enforceability may be subject to, in each case, (i) the exceptions and limitations in New York General Obligations Law sections 5-1401 and 5-1402 and (ii) principles of comity and constitutionality.

In addition, in rendering the foregoing opinions we have assumed that, at all applicable times:

(a)    neither the execution and delivery by the Company of the Transaction Documents nor the performance by the Company of its obligations thereunder, including the issuance and sale of the Securities: (i) constitutes or will constitute a violation of, or a default under, any lease, indenture, agreement or other instrument to which the Company or its property is subject (except that we do not make the assumption set forth in this clause (i) with respect to those agreements or instruments expressed to be governed by the laws of the State of New York which are listed in the Company’s most recent Annual Report on Form 10-K), (ii) contravenes or will contravene any order or decree of any governmental authority to which the Company or its property is subject, or (iii) violates or will violate any law, rule or regulation to which the Company or its property is subject (except that we do not make the assumption set forth in this clause (iii) with respect to the Opined on Law); and

(b)    neither the execution and delivery by the Company of the Transaction Documents nor the performance by the Company of its obligations thereunder, including the issuance and sale of the Securities, requires or will require the consent, approval, licensing or authorization of, or any filing, recording or registration with, any governmental authority under any law, rule or regulation of any jurisdiction.

We hereby consent to the reference to our firm under the heading “Validity of Securities” in the Preliminary Prospectus and the Prospectus. In giving this consent, we do not thereby admit that we are within the category of persons whose consent is required under Section 7 of the Securities Act or the Rules and Regulations. We also hereby consent to the filing of this opinion with the Commission as an exhibit to the Company’s Current Report on Form 8-K being filed on

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the date hereof and incorporated by reference into the Registration Statement. This opinion is expressed as of the date hereof unless otherwise expressly stated, and we disclaim any undertaking to advise you of any subsequent changes in the facts stated or assumed herein or of any subsequent changes in applicable laws.