Mitchum initially appeared unbilled in (e.g., Flying Leathernecks, RKO 1951) and extra roles before gradually receiving bigger character parts.

He supported his more famous brother on several occasions, and became known as the friendly, food-loving Inspector Frank DiGiorgio in the first three Dirty Harry films.

In 1958, Mitchum was cast in two episodes of the crime drama Richard Diamond, Private Detective, starring David Janssen.

[citation needed] On September 15, 1959, Mitchum appeared in the premiere episode "Stage Stop" of the Western series Laramie.

[citation needed] Mitchum was cast in 1960 as Pickalong in 10 episodes of the Western series Riverboat, starring Darren McGavin.

From 1965 to 1967, Mitchum had the recurring role of Trooper Hoffenmueller in 11 episodes of the sitcom F Troop, starring Forrest Tucker, Larry Storch, and Ken Berry.