Additional Proxy Soliciting Materials (Form DEFA14A)

Your Vote Counts! Smartphone users Point your camera here and vote without entering a control number For complete information and to vote, visit www.ProxyVote.com Control # V81981-P41695 AMENTUM HOLDINGS, INC. 4800 WESTFIELDS BLVD, SUITE 400 CHANTILLY, VA 20151 AMENTUM HOLDINGS, INC. You invested in AMENTUM HOLDINGS, INC. and it's time to vote! You have the right to vote on proposals being presented at the Annual Meeting. This is an important notice regarding the availability of proxy materials for the stockholder meeting to be held on February 6, 2026. Vote Virtually at the Meeting* February 6, 2026 9:00 a.m. (EST) Virtually at: www.virtualshareholdermeeting.com/AMTM2026 *Please check the meeting materials for any special requirements for meeting attendance. Get informed before you vote View the Proxy Statement and Form 10-K online OR you can receive a free paper or email copy of the material(s) by requesting prior to January 23, 2026. If you would like to request a copy of the material(s) for this and/or future stockholder meetings, you may (1) visit www.ProxyVote.com, (2) call 1-800-579-1639 or (3) send an email to [email protected]. If sending an email, please include your control number (indicated below) in the subject line. Unless requested, you will not otherwise receive a paper or email copy. 2026 Annual Meeting Vote by February 5, 2026 11:59 PM EST. For shares held in a Plan, vote by February 3, 2026 11:59 PM EST.

Vote at www.ProxyVote.com Prefer to receive an email instead? While voting on www.ProxyVote.com, be sure to click "Delivery Settings". Voting Items Board Recommends V81982-P41695 THIS IS NOT A VOTABLE BALLOT This is an overview of the proposals being presented at the upcoming stockholder meeting. Please follow the instructions on the reverse side to vote these important matters. 1. Election of Directors Nominees: 1a. Steven J. Demetriou For 1b. John Heller For 1c. Benjamin Dickson For 1d. Vincent K. Brooks For 1e. Ralph E. Eberhart For 1f. Alan E. Goldberg For 1g. S. Leslie Ireland For 1h. Barbara L. Loughran For 1i. Sandra E. Rowland For 1j. Christopher M.T. Thompson For 1k. Russell Triedman For 1l. John Vollmer For 1m. Connor Wentzell For 2. The ratification of the appointment of Ernst & Young LLP as the Company's independent registered public accounting firm for the Company's fiscal year 2026. For 3. Advisory vote to approve the Company's named executive officer compensation. For NOTE: Such other business as may properly come before the meeting or any adjournment thereof.