He appeared in a number of British and Italian films in the mid-1950s, including Federico Fellini's La Strada and Il Bidone.

He worked as a reporter at his father's newspaper and as a radio announcer in Zanesville and Columbus, Ohio, before entering a stage career at the Hedgerow Theatre in Pennsylvania.

He won the 1945 New York Drama Critics' Circle Award for Most Promising Young Actor for his starring role in John Patrick's play The Hasty Heart, which was adapted into a 1949 film of the same name.

[4] He soon appeared as the killer in the film noir classic He Walked by Night (1948) for Eagle-Lion, then he appeared as a psychotic member of the Hatfield clan in Roseanna McCoy (1949), as Maximilien Robespierre in the period film noir Reign of Terror (1949), as a timid husband in Tension (1950), as Ishmael in Moby Dick (1956), in the drama Decision Before Dawn (1951), George S. Healey in Titanic (1953) and as Ivan in The Brothers Karamazov (1958).

[6] From 1964 to 1968, Basehart played the lead role, Admiral Harriman Nelson, on Irwin Allen's first foray into science-fiction television, Voyage to the Bottom of the Sea.

He appeared in "Probe 7, Over and Out", an episode of The Twilight Zone,[8] Hawaii Five-O, and as Hannibal Applewood, an abusive schoolteacher in Little House on the Prairie in 1976.

In 1972, Basehart appeared in the Columbo episode "Dagger of the Mind", in which Honor Blackman and he played a husband-and-wife theatrical team who accidentally kill Sir Roger Haversham, the producer of their rendition of Macbeth.

After the death of his first wife Stephanie Klein, he married Italian actress Valentina Cortese, with whom he had one son, actor Jackie Basehart; the couple divorced in 1960.