He initially appeared in only unbilled and extra roles before gradually receiving bigger character parts in middle age. Mitchum supported his more famous brother on several occasions, and was most famous in his own right as the friendly and food-loving Inspector Frank DiGiorgio in the first three Dirty Harry films. Mitchum was one of only four actors to appear in more than one film in this series (the others being Clint Eastwood, Harry Guardino and Albert Popwell), and along with Eastwood and Guardino, was one of the only three actors to play the same character in each appearance, until his character was killed off in the third film, The Enforcer. In 1958, Mitchum was cast in two episodes of the CBS crime drama, Richard Diamond, Private Detective, starring David Janssen. He played Joe in the episode “Short Haul” and Jimmy Logan in “Bungalow Murder”. On September 15, 1959, Mitchum appeared in the premiere episode “Stage Stop” of NBC’s Laramie western series. The episode explains how series characters Slim Sherman and Jess Harper, played by John Smith and Robert Fuller, respectively, become partners in the Sherman Ranch and Relay Station. Jess arrives in Wyoming from Texas in search of an erstwhile “friend”, Pete Morgan, played by John Mitchum, who had robbed Jess. Morgan is part of the gang of Bud Carlin, played by Dan Duryea. The gang captures Judge Thomas J. Wilkens, portrayed by Everett Sloane, to keep him from trying Morgan. Though Jess and Slim are at odds with each other in their first encounters, and friendshipn seems out of the question, Andy Sherman, played by Robert L. Crawford, Jr., takes an instant liking to Jess and ask Jess to take him away from the ranch, where he lives with his older brother Slim. Slim and Jess must fight together when Carlin shows up at the relay station and proceeds to humiliate the judge.

Mitchum was cast in 1960 as the character with the unusual name, “Pickalong”, in ten episodes of another NBC western, Riverboat, starring Darren McGavin. That same year, he appeared in the ABC western series, The Rebel, starring Nick Adams. He also appeared as Hal Swanson in the 1960 episode “Silver Killers” of the western series, Tombstone Territory. From 1965 to 1967, Mitchum held the recurring role of Trooper Hoffenmueller in eleven episodes of the ABC military sitcom, F Troop, starring Forrest Tucker, Larry Storch, and Ken Berry. Mitchum was a writer, poet, singer, and guitar player. An autobiography/biography about his life and career and that of his brother, Robert Mitchum, was published in the 1998 called Them Ornery Mitchum Boys. He composed the piece “America, Why I Love Her”, which John Wayne included in his book and album of the same name. The piece and a short film with Wayne’s narration were aired at many television stations at sign-off time before stations began broadcasting 24 hours a day in late 1970s and early 1980s. Wayne is often mistakenly credited with composing the piece. Mitchum died of a stroke, at the age of 82. Mitchum also served in the United States Merchant Marine.