In the 1960s and 1970s, Mustin appeared in The Adventures of Huckleberry Finn (1960), Hemingway's Adventures of a Young Man (1962), Twilight of Honor (1963), What a Way to Go! (1964), The Misadventures of Merlin Jones (1964), Sex and the Single Girl (1964), The Cincinnati Kid (1965), Cat Ballou (1965) (uncredited as a former gunfighter "Old ... Old ... ?" ), The Ghost and Mr. Chicken (1965), The Adventures of Bullwhip Griffin (1967), Speedway (uncredited) (1968), The Shakiest Gun in the West (1968), The Great Bank Robbery (1969), Hail, Hero! (1969), and Skin Game (1971). In 1974, Mustin portrayed "Uncle Jeff" in the musical film Mame, starring Lucille Ball and Bea Arthur. He also had a small role in Herbie Rides Again, also released in 1974. The next year, he appeared as "Regent Appleby" in The Strongest Man in the World. His final film role came in 1976 in the Western film Baker's Hawk, starring Clint Walker and Burl Ives.