Leo Carroll was born in Weedon Bec, Northamptonshire, to William and Catherine Carroll. His Roman Catholic parents named him after then-Pope Leo XIII. In 1897, his family lived in York, where his Irish-born father was a foreman in an ordnance store. In the 1901 Census for West Ham, London, his occupation is listed as “wine trade clerk”. In the 1911 census, he is living at the same address and described as a “dramatic agent”. Carroll made his stage debut in 1912. His acting career was on hold during the First World War, when he served in the British Army. He then performed in London and on Broadway in New York City. In 1933, he was a member of the Manhattan Theatre Repertory Company in the inaugural season of the Ogunquit Playhouse in Ogunquit, Maine. During 1933-34 Carroll had an important role in a successful Broadway play, The Green Bay Tree (which has no relation besides the shared title to the novel by Louis Bromfield), and in 1941 starred with Vincent Price and Judith Evelyn in Patrick Hamilton’s Angel Street (Gas Light), which ran for three years at the Golden Theatre on West 45th Street in New York City. After the production closed, he starred in the title role in J. P. Marquand’s The Late George Apley.

Carroll, who had moved to Hollywood, made his film debut in Sadie McKee (1934). He often played doctors or butlers, but he made appearances as Marley’s ghost in A Christmas Carol (1938) and as Joseph in Wuthering Heights (1939). In the original version of Father of the Bride (1950), he played an unctuous wedding caterer. In the 1951 film The Desert Fox: The Story of Rommel he played a sympathetic German General Gerd von Rundstedt, presenting him as a tragic, resigned figure completely disillusioned with Hitler. Carroll is perhaps best known for his roles in six Alfred Hitchcock films: Rebecca (1940), Suspicion (1941), Spellbound (1945), The Paradine Case (1947), Strangers on a Train (1951), and North by Northwest (1959). He appeared in more Hitchcock films than anyone other than Clare Greet (1871–1939) (who appeared in seven) and Hitchcock himself, whose cameos were a trademark. As with earlier roles, he was often cast as doctors or other authority figures (such as the spymaster “Professor” in North by Northwest). In addition to appearing as Rev. Mosby with actress Hayley Mills in The Parent Trap (1961), Carroll is remembered for his role as the frustrated banker haunted by the ghosts of George and Marion Kerby in the television series Topper (1953–1956), with costars Anne Jeffreys, Robert Sterling, and Lee Patrick. He appeared as the older Father Fitzgibbon from 1962 to 1963 in ABC’s Going My Way”‘, a series about two Roman Catholic priests at St. Dominic’s parish in New York City. Gene Kelly held the lead as Father Chuck O’Malley, with Dick York as Tom Colwell, who operates a neighborhood youth center. Carroll subsequently starred as spymaster Alexander Waverly on The Man from U.N.C.L.E. (1964–1968). Several U.N.C.L.E. films followed, and a spin-off television series, The Girl from U.N.C.L.E. in 1966. He was one of the first actors to appear in two different television series as the same character. In 1972, Carroll died in Hollywood of cancer-induced pneumonia. He is interred at the Grand View Memorial Park Cemetery in Glendale, California.