X0202 SCHEDULE 13D/A 0001140361-25-046593 0000225602 XXXXXXXX LIVE 1 Ordinary Shares, $1.50 per share par value 06/30/2026 0001274173 G4474Y214 Janus Henderson Group plc

201 Bishopsgate London X0 EC2M 3AE

Jaime Genua (800) 767-1000 Massachusetts Mutual Life Insurance Co 1295 State Street Springfield MA 01111 0000225602 N Massachusetts Mutual Life Insurance Company b WC MA 0 0 0 0 0 0 IC Ordinary Shares, $1.50 per share par value Janus Henderson Group plc 201 Bishopsgate London X0 EC2M 3AE This Amendment No. 1 ("Amendment No. 1") amends and supplements the Schedule 13D filed with the SEC on December 23, 2025 (the "Schedule 13D"), relating to the Ordinary Shares, $1.50 per share par value (the "Ordinary Shares"), of Janus Henderson Group plc, a public limited company organized under the Laws of Jersey (the "Issuer"). Capitalized terms not defined herein shall have the meaning ascribed to them in the Schedule 13D. Except as set forth herein, the Schedule 13D is unmodified. Items 4, 5, and 6 of the Schedule 13D are hereby amended as follows: Item 4 of the Schedule 13D is hereby amended and supplemented by adding the following: On June 30, 2026, the transactions contemplated by the Merger Agreement and the Equity Commitment Letter were consummated. As a result of the Merger, the Ordinary Shares will no longer be listed on The New York Stock Exchange and will be deregistered under Section 12(b) of the Exchange Act. Additionally, as a result of the Merger, MassMutual no longer beneficially owns any Ordinary Shares, though MassMutual retains an equity interest in the surviving company through its beneficial ownership of preferred equity interests of Topco. Item 5(a) is hereby amended and restated in its entirety as follows: As of June 30, 2026, MassMutual no longer beneficially owned any Ordinary Shares of the Issuer. Item 5(b) is hereby amended and restated in its entirety as follows: As of June 30, 2026, MassMutual no longer beneficially owned any Ordinary Shares of the Issuer. Item 5(c) is hereby amended and restated in its entirety as follows: MassMutual has not effected any transactions in the Ordinary Shares during the past 60 days other than as described in this Schedule 13D and as set forth on Schedule II attached hereto. Item 5(e) is hereby amended and restated in its entirety as follows: As of June 30, 2026, upon consummation of the Merger, MassMutual may no longer deemed to be part of a group that beneficially owned more than five percent of the outstanding Ordinary Shares of the Issuer. Neither the filing of this Schedule 13D nor any of its contents shall be deemed to constitute an admission that MassMutual is or was a member of any such group. Item 6 of the Schedule 13D is hereby supplemented to incorporate by reference the information set forth in Item 4 above. Exhibit 99.3 of the Schedule 13D is hereby amended and restated in its entirety as Exhibit 99.1 attached to this Amendment No. 1. Exhibit 99.1 Schedule II to Schedule 13D Massachusetts Mutual Life Insurance Company /s/ Jaime Genua 07/02/2026