C.J. Cregg becomes White House Chief of Staff in season 6 of The West Wing. Allison Janney’s character had served as President Bartlet’s Press Secretary from the beginning of the series, making the promotion a major change in both her responsibilities and the show’s power structure.
The transition follows Leo McGarry’s departure from the chief-of-staff position during the sixth season. After a crisis connected to the Camp David peace negotiations, Leo suffers a near-fatal heart attack and later moves into the role of Senior Counselor to the President. Bartlet then appoints C.J. as his successor.
C.J.’s first days in the new position are shown in the season-six episode “Liftoff.” She must manage the entire senior staff, protect the President’s schedule, and make decisions that previously belonged to Leo. Her promotion also places a woman at the center of the administration’s daily operations.
The answer can be confused with season 7 because C.J. remains chief of staff during that season. However, the promotion itself occurs in season 6, and she holds the job through the end of the Bartlet administration.