John Dall Thompson (he used his middle name for his acting career) was born in New York City, the younger son of Charles Jenner Thompson and his wife Henry (née Worthington). His father was a civil engineer. His elder brother, Worthington Thompson, was later a decorated lieutenant in the 517th Parachute Regimental Combat Team.
In the 1920s the Thompsons moved to Panama, where Charles worked on the construction of the airport there. While in Panama Dall was in a theatre when a cast member fell ill and Dall stepped in; this caused him to get the acting bug. His father committed suicide in 1929, and his widow returned to New York City with John the following year.
John attended Horace Mann School and briefly enrolled at Columbia University, where he intended to follow in his father's footsteps by studying engineering. Deciding that acting was his true vocation, he left Columbia and studied at the Theodora Irvine School of Theater and the Pasadena Playhouse. He also studied at the Petit Theatre in New Orleans.
Dall spent six years acting in various stock companies, notably Claire Tree Major's Children's Theatre. He also worked in companies headed by Aline MacMahon, Arthur Byron, Ruth Weston and Edith Atwater. Dall had small roles on Broadway during the 1941 - 42 season in R.U.R. and Janie . In 1942 - 43 he played the lead of Quizz Martin in the touring company The Eve of St Mark . He was well received and replaced William Price on Broadway so Price could take a vacation. His performance was spotted by Jack Warner's wife and resulted in a Warners screen test for Dall. Warners offered him a contract but he would only take it if he could have time off to do a play.
Dall's first film in eight years was Spartacus (1960), where he played a Roman soldier. He was the villain in MGM's Atlantis, the Lost Continent (1961), his final feature. Dall guest starred in a number of episodes of Perry Mason , playing different characters.
John Dall, primarily a stage actor, is best remembered today for two film roles: the cool-minded intellectual killer in Alfred Hitchcock's Rope (1948), and the trigger-happy lead in the 1950 noir Gun Crazy . He first came to fame as the young prodigy who comes alive under the tutelage of Bette Davis in The Corn Is Green (1945), for which he was nominated for the Academy Award for Best Supporting Actor.
Film historians William J. Mann and Karen Burroughs Hannsberry have remarked that Dall was gay but claimed in media interviews to have had a brief marriage in the early 1940s. No marriage certificate has come to light, and his death certificate records him as "never married". Hedda Hopper once linked his name with Jane Withers romantically. According to music journalist Phil Milstein, at the time of his death Dall had lapsed into alcoholism and was living with his partner, actor Clement Brace (died 1996).
Dall sustained a serious fall while visiting London in October 1970, and died of a heart attack, a complication of an infection of the pericardium at his home in Beverly Hills, California, aged 50. His body was donated to medical science.