Amentum Holdings, Inc.

UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

Proxy Statement Pursuant to Section 14(a) of

the Securities Exchange Act of 1934 (Amendment No. )

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Preliminary Proxy Statement

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Definitive Proxy Statement

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Definitive Additional Materials

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Soliciting Materials Pursuant to §240.14a-12

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Notice of 2026

Annual Meeting

of Stockholders

and Proxy Statement​

February 6, 2026

Amentum Holdings, Inc.

Dear Amentum Stockholder:

I would like to invite you to attend the Annual Meeting of Stockholders of Amentum Holdings, Inc. (“Amentum” or the “Company”), to be held virtually at 9:00 a.m. (EST) on February 6, 2026 (the “Annual Meeting”). A Notice of Internet Availability of Proxy Materials, which contains instructions on how to access online our Proxy Statement and our Annual Report for the fiscal year ended October 3, 2025, is first being sent to stockholders on or about December 19, 2025.

Items of business to be transacted at our Annual Meeting are:

Election of thirteen director nominees;

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Ratification of the appointment of Ernst & Young LLP as our independent registered public accounting firm for fiscal year 2026; and

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A non-binding, advisory vote on the compensation paid to the Company’s named executive officers in fiscal year 2025, as disclosed in the proxy statement; and

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In addition to the foregoing, the Annual Meeting will include the transaction of such other business as may properly come before the meeting, or any adjournment(s), continuation(s), rescheduling(s) or postponement(s) thereof.

The Board of Directors recommends that you vote “FOR” the nominees under Proposal 1, and “FOR” Proposals 2 and 3.

Our 2026 Annual Meeting of Stockholders will be a virtual meeting conducted solely online and can be attended by visiting www.virtualshareholdermeeting.com/AMTM2026. To participate in the Annual Meeting, you will need the control number located on your proxy card or the instructions that accompanied your proxy materials.

Your vote is important. You may vote by proxy online or by telephone. In addition, if you receive paper copies of the proxy materials by mail, you may also vote by mail by following the instructions on the proxy card or voting instruction form (if you hold your shares through a broker or bank). Voting online, by telephone, by written proxy or by voting instruction form will ensure your representation at the Annual Meeting regardless of whether you participate in the virtual meeting.

We look forward to your participation in our Annual Meeting. We appreciate your investment in and support of Amentum.

Sincerely,

John Heller

Chief Executive Officer

NOTICE OF AMENTUM HOLDINGS, INC.’S 2026 ANNUAL MEETING OF STOCKHOLDERS

• Our 2026 Annual Meeting of Stockholders will be a virtual meeting conducted solely online and can be attended by visiting www.virtualshareholdermeeting.com/AMTM2026. To participate in the Annual Meeting, you will need the control number located on your proxy card or the instructions that accompanied your proxy materials. If you plan to participate in the virtual meeting, please see “Important Information About Annual Meeting and Proxy Procedures.”

On Behalf of the Board of Directors,

Michele T. St. Mary

Chief Legal Officer, General

Counsel & Corporate Secretary

Important Notice Regarding the Availability of Proxy Materials for the Annual Meeting of Stockholders to be held on February 6, 2026: On December 19, 2025, we will begin mailing a notice, called the Notice of Internet Availability of Proxy Materials (the “Notice”), to our stockholders advising them that this Proxy Statement, our Annual Report on Form 10-K for the fiscal year ended October 3, 2025 and voting instructions can be accessed over the Internet at www.proxyvote.com. You may then access these proxy materials online or you may request that a printed copy of the materials be sent to you. If you want to receive a paper or e-mail copy of these proxy materials, you may request one by telephone by calling 1-800-579-1639, online at www.proxyvote.com or by e-mail to sendmaterial@proxyvote.com. If sending an email, please include your control number (indicated on your Notice) in the subject line. There is no charge to you for requesting a copy.