Willdan Group, Inc.

Item 1.01 Entry into a Material Definitive Agreement The information related to the Amendment (as defined below) set forth under Item 2.03 of this Current Report on Form 8-K is hereby incorporated herein by reference.

Item 2.01 Completion of Acquisition or Disposition of Assets As previously reported, on September 22, 2026, Willdan Group, Inc. (the “Company”), through its wholly owned subsidiary, Willdan Energy Solutions, Inc. (“Energy Solutions”), entered into that certain Membership Interest Purchase Agreement, among Mantis Intermediate Holdings, LLC (“Mantis”), Mantis NewCo, LLC (“Seller”), Willdan and Energy Solutions (the “Purchase Agreement”) to acquire one hundred percent of the issued and outstanding equity securities of Mantis (the “Transaction”). On October 30, 2026, the Company completed the Transaction and, pursuant to the Purchase Agreement, Energy Solutions acquired all the equity interests of Mantis. Pursuant to the Purchase Agreement, Energy Solutions paid approximately $285,000,000, subject to certain customary post-closing adjustments. The Company paid the purchase price for the Transaction using a combination of $35.0 million of borrowings under its existing Revolving Credit Facility and $250.0 million of borrowings under its Credit Agreement, as described below. The foregoing description of the Purchase Agreement does not purport to be complete and is qualified in its entirety by reference to the Purchase Agreement, which was filed as Exhibit 2.1 on the Current Report on Form 8-K filed by the Company on September 23, 2026 and is incorporated herein by reference. Certain schedules and exhibits to the Purchase Agreement were omitted pursuant to Item 601(b)(2) of Regulation S-K. The Company agrees to furnish supplementally a copy of any omitted schedule or exhibits to the Securities and Exchange Commission upon request.

Item 2.03. Creation of a Direct Financial Obligation or an Obligation under an Off-Balance Sheet Arrangement of a Registrant On October 30, 2026, the Company entered into an Increase Request and Second Amendment to Amended and Restated Credit Agreement (the “Amendment”) among the Company, certain subsidiaries of the Company party thereto, the Lenders (as defined below) party thereto, and BMO Harris Bank, N.A., as administrative agent (in such capacity, the “Agent”), which amends the Company’s existing Amended and Restated Credit Agreement, dated as of May 5, 2025 (as amended, supplemented, or otherwise modified prior to the date of the Amendment, the “Existing Credit Agreement”; the Existing Credit Agreement as amended by the Amendment, the “Credit Agreement”), among the Company, the subsidiaries of the Company from time to time party thereto, the financial institutions from time to time party thereto as lenders (the “Lenders”), and the Agent. The Existing Credit Agreement provided for up to a $50.0 million term loan facility (the “Existing Term Loan Facility”), up to a $50.0 million delayed draw term loan facility (the “Existing Delayed Draw Term Loan Facility”) and a $100.0 million revolving credit facility (the “Existing Revolving Credit Facility”; together with the Existing Term Loan Facility and the Existing Delayed Draw Term Loan Facility, collectively, the “Existing Credit Facilities”), each maturing on May 5, 2030. The Amendment amended the Existing Credit Agreement to, among other things, add an additional up to $250.0 million term loan facility (the “New Term Loan Facility”) to be used to finance the Transaction and to pay certain related fees and expenses, including, without limitation, to repay certain existing indebtedness of Mantis. The New Term Loan Facility matures on May 5, 2030 and bears interest at rates, and is subject to customary covenants, identical to those applicable to the Existing Credit Facilities, as described in the Credit Agreement. Except as expressly amended by the Amendment, the Existing Credit Agreement remains in effect in accordance with its existing terms. On October 30, 2026, in connection with the closing of the Transaction, the Company borrowed $250.0 million under the New Term Loan Facility. The proceeds of such borrowings were used to pay part of the consideration owed in connection with the Transaction along with related fees and expenses, including, without limitation, to repay certain existing indebtedness of Mantis. The foregoing description of the Amendment does not purport to be complete and is qualified in its entirety by reference to the Amendment filed as Exhibit 10.1 on this Current Report on Form 8-K and incorporated herein by reference. Certain schedules and exhibits to the Amendment were omitted pursuant to Item 601(b)(2) of Regulation S-K. The Company agrees to furnish supplementally a copy of any omitted schedule or exhibits to the Securities and Exchange Commission upon request.

Item 7.01. Regulation FD Disclosure On October 30, 2026, the Company issued a press release announcing the closing of the Transaction. A copy of the press release is attached as Exhibit 99.1 hereto and is hereby incorporated by reference in its entirety. The information in this Item 7.01 is being furnished and shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or incorporated by reference in any filing under the Securities Act, or the Exchange Act, except as shall be expressly set forth by specific reference in such a filing.