Company: MGRE
Filing Date: 2025-02-28
Form Type: S-3ASR
Source: 0001193125-25-042543
Chunk: 1

Company: AFFILIATED MANAGERS GROUP, INC.
Filing Date: 2025-02-28
Form: S-3ASR
Chunk 1
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 Securities Act, check the following box. ☒ If this Form is a post-effective amendment to a registration statement filed pursuant to General Instruction I.D. filed to register additional securities or additional classes of securities pursuant to Rule 413(b) under the Securities Act, check the following box. ☐ Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-acceleratedfiler, a smaller reporting company, or an emerging growth company. See the definitions of “large accelerated filer,” “accelerated filer,” “smaller reporting company” and “emerging growth company” in Rule 12b-2of the Exchange Act.

| Large accelerated filer |     | ☒ |     | Accelerated filer         |     | ☐ |
| Non-accelerated filer   |     | ☐ |     | Smaller reporting company |     | ☐ |
|                         |     |   |     | Emerging Growth company   |     | ☐ |

If an emerging growth company, indicate by check mark if the registrant has elected not to use the extended transition period for complying with any new or revised financial accounting standards provided pursuant to Section 7(a)(2)(B) of Securities Act. ☐

PROSPECTUS

AFFILIATED MANAGERS GROUP, INC.

Debt Securities

Common Stock

Preferred Stock

Depositary Shares

Warrants

Subscription Rights

Stock Purchase Contracts

Stock Purchase Units

The registration statement that
contains this prospectus is being filed in anticipation of the expiration of the previous registration statement filed by Affiliated Managers Group, Inc. (“AMG”) on March 1, 2022.

AMG may offer and sell, or facilitate the resale of, securities from time to time. We will provide specific terms of these securities in supplements to this
prospectus. One or more selling security holders to be identified in the future may also offer and sell the securities listed above from time to time. You should read this prospectus and any prospectus supplements carefully before making your
investment decision.

The common stock of AMG is listed on the New York Stock Exchange (the “NYSE”) under the symbol “AMG.” In
addition, we have junior subordinated notes listed on the NYSE under the symbols “MGR,” “MGRB,” “MGRD,” and “MGRE.” If we decide to seek a listing of any debt securities, preferred stock, depositary