When Nixon returned to La Casa Pacifica in San Clemente, California, nicknamed "The Western White House" during his administration, Brennan left the Marines and served as the ex-president's chief of staff.
In the role of chief of staff, Brennan managed the small staff, appointments, mail and budgets, and was the liaison to the General Services Administration and members of the United States Congress who wanted information for any ongoing investigations from the former President's Administration.