first television appearance on Armstrong Circle Theater in the episode The Jailbreak. He appeared regularly on television as a guest actor during the 1960s, often in action, suspense, detective, or crime dramas. His appearances during this time include performances on Alfred Hitchcock Presents, Naked City, The Untouchables, Route 66, The Twilight Zone, The Outer Limits, The Fugitive, T.H.E. Cat, Voyage to the Bottom of the Sea, The Time Tunnel and The Mod Squad.
Duvall's screen debut was as Boo Radley in the critically acclaimed To Kill a Mockingbird (1962). He was cast in the film on the recommendation of screenwriter Horton Foote, who met Duvall at Neighborhood Playhouse during a 1957 production of Foote's play, The Midnight Caller. Foote, who would collaborate with Duvall many more times over the course of their careers, said he believed Duvall had a particular love of common people and ability to infuse fascinating revelations into his roles. Foote has described Duvall as "our number one actor."
After To Kill a Mockingbird, Duvall appeared in a number of films during the 1960s, mostly in mid sized parts but also in a few larger supporting roles. Some of his more notable appearances include the role of Capt. Paul Cabot Winston in Captain Newman, M.D. (1963), Chiz in Countdown (1968), and Gordon in The Rain People. Duvall has a small part as a cab driver who ferries McQueen around just before the chase scene in the film Bullitt (1969). He was the notorious malefactor "Lucky" Ned Pepper in True Grit (1969), in which he engaged in a climactic shootout with John Wayne's Rooster Cogburn on horseback.