Leo McGarry served as White House Chief of Staff in the first season of The West Wing. John Spencer played Leo, the senior adviser who helps President Josiah Bartlet run the fictional Democratic administration.
Leo is more than an administrator: he is Bartlet's longtime friend, political strategist, and trusted gatekeeper. Before joining the Bartlet White House, he had served as Secretary of Labor and had extensive experience in Democratic politics. His authority explains why the other senior staff members—Josh Lyman, Toby Ziegler, C.J. Cregg, and Sam Seaborn—report into a tightly coordinated chain of command.
A frequent mix-up is confusing Leo with Josh Lyman. Josh is the Deputy Chief of Staff for most of the series, while Leo holds the top staff position during seasons one through six. C.J. Cregg is the Press Secretary, and Toby Ziegler is the Communications Director, so neither is the first-season Chief of Staff.
Leo's role also anchors some of the show's most serious stories, including his recovery from alcoholism and his efforts to balance personal loyalty with presidential responsibility. He remains one of the administration's central figures until John Spencer's death during the show's final season.