A descendant of Supreme Court Justice John Marshall, this veteran of over 40 films is best remembered as the star of "Daktari", a popular family TV series set in Africa that ran on CBS from January 1966 to January 1969. The inspiration for the show was the feature film "Clarence the Cross-Eyed Lion" (1964) which Thompson starred in and co-wrote.

Cast in naive juvenile roles in MGM films of the 1940s, Thompson began playing a wide variety of roles as a mature lead or second lead. He appeared in Vincente Minnelli's "The Clock" (1945), John Ford's "They Were Expendable" (1945), William Wellman's "Battleground" (1949), Anthony Mann's "Devil's Doorway" (1950) and "The Tall Target" (1951), Herbert Ross' "The Turning Point" (1977) and Sam Fuller's "White Dog" (1982). He also faced down the uncanny in the likes of "Fiend Without a Face", "It! The Terror From Beyond Space" and "First Man in Space". Thompson spent much of his time in Africa in his later years, where he produced, directed, and acted in the documentary series "Orphans of the Wild".