In 2254, following the encounter made by the USS Enterprise with the planet Talos IV, under Captain Christopher Pike, he was the primary officer involved in issuing Starfleet General Order 7, as evidenced by his signing off on the document.
"Commodore Mendez. Urgent. Subspace monitors show Enterprise receiving transmissions from Planet Talos IV, in violation of Starfleet General Orders. Captain Kirk is hereby relieved. You are ordered to assume command of the Enterprise. Disable vessel if necessary to prevent further contact." (Signed Comsol, Starfleet Command)
Spock argued with Mendez that Comsol's order to relive Kirk, despite Kirk knowing nothing of the Talos connect, Mendez reminded Spock that as long as Kirk commanded the Enterprise, he was responsible for everything that happens aboard his ship. (TOS: "The Menagerie, Part I")