Custom Truck One Source, 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.)

Filed by the Registrantx

Filed by a Party other than the Registranto

Check the appropriate box:

(Name of Registrant as Specified in its Charter)

(Name of Person(s) Filing Proxy Statement, if Other Than the Registrant)

Payment of Filing Fee (Check all boxes that apply):

7701 Independence Ave

April 28, 2026

Dear Custom Truck One Source Stockholder:

On behalf of the Board of Directors and management of Custom Truck One Source, Inc., I invite you to attend remotely the 2026 Annual Meeting of Stockholders (the "Annual Meeting").

The Annual Meeting will be held on Thursday, June 11, 2026 at 9:00 a.m. Eastern Time. It will be conducted via live webcast only, and you will be able to participate online at https://www.virtualshareholdermeeting.com/CTOS2026. You may submit questions during the meeting and will be able to vote your shares electronically. The accompanying Notice of Annual Meeting of Stockholders and Proxy Statement include details regarding how to join the Annual Meeting and the business to be conducted at the Annual Meeting.

I hope that you will be able to join the Annual Meeting. If you do not plan to attend, I strongly encourage you to vote as soon as possible to ensure that your shares are represented at the Annual Meeting. The accompanying Proxy Statement explains more about voting. Please read it carefully.

We sincerely appreciate your continued support.

Sincerely,

Chairman of the Board

7701 Independence Ave

NOTICE OF ANNUAL MEETING OF STOCKHOLDERS

To Be Held at 9:00 a.m. Eastern Time on Thursday, June 11, 2026

Dear Stockholders of Custom Truck One Source, Inc.:

We cordially invite you to attend the 2026 annual meeting of stockholders (the "Annual Meeting") of Custom Truck One Source, Inc., a Delaware corporation, which will be held on Thursday, June 11, 2026 at 9:00 a.m. Eastern Time, for the following purposes, as more fully described in the accompanying proxy statement:

1. To elect three Class A directors to serve until the 2029 annual meeting of stockholders and until their successors are duly elected and qualified;

2. To ratify the appointment of Ernst & Young LLP as our independent registered public accounting firm for our fiscal year ending December 31, 2026; and

3. To transact such other business as may properly come before the Annual Meeting or any adjournments or postponements thereof.

The Annual Meeting will be conducted via live webcast, and you will be able to participate online at https://www.virtualshareholdermeeting.com/CTOS2026. You may submit questions during the meeting and will be able to vote your shares electronically.

Our board of directors has fixed the close of business on April 16, 2026 as the record date for the Annual Meeting, or any continuation, postponement or adjournment of the Annual Meeting. A complete list of such stockholders will be open to the examination of any stockholder for a period of 10 days prior to the Annual Meeting for a purpose germane to the Annual Meeting at the Company’s offices at 7701 Independence Ave., Kansas City, Missouri 64125. The list of these stockholders will also be available to view during the Annual Meeting for stockholders who attend the meeting. The Annual Meeting may be continued or adjourned from time to time without notice other than by announcement at the Annual Meeting. We will first make our proxy statement, proxy card and other proxy materials available to stockholders on or about April 28, 2026. Only stockholders of record on April 16, 2026 are entitled to notice of and to vote at the Annual Meeting. Further information regarding voting rights and the matters to be voted upon is presented in the accompanying proxy statement.

YOUR VOTE IS IMPORTANT. We urge you to submit your vote via the Internet, telephone or mail as soon as possible to ensure that your shares are represented, regardless of whether you plan to attend the Annual Meeting. For additional instructions on voting by the Internet, please refer to your proxy card. Returning the proxy does not deprive you of your right to attend the Annual Meeting electronically and to vote your shares at the Annual Meeting.

We sincerely appreciate your continued support.

By order of the Board of Directors,

Executive Vice President, General Counsel and Corporate Secretary

Kansas City, Missouri

April 28, 2026

Table of Contents

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PROXY STATEMENT

FOR

2026 ANNUAL MEETING OF STOCKHOLDERS