Born to a wealthy English Catholic family, he was named after the reigning Pope at the time of his birth, Leo XIII (1810 - 1903, reigned 1878 - 1903)
Fought in the British army during WW I and was seriously wounded.
He appeared as Laurence Olivier's manservant in The Green Bay Tree at Broadway's Cort Theatre in the 1933-34 season, in which Olivier co-starred with his real-life first wife, Jill Esmond. The play, directed by the legendary Jed Harris, was a hit, playing for 166 performances. The Green Bay Tree, written by Mordaunt Shairp, was one of the first plays to deal with the topic of homosexuality.
He was mentioned in the song "Science Fiction/Double Feature" from The Rocky Horror Picture Show (1975).
Made his Broadway debut in K.G. Sowerby's play Rutherford & Son at the Little Theatre on December 24, 1912. He last appeared on the Great White Way over 40 years later in Emlyn Williams's Someone Waiting at the John Golden Theatre, a flop which opened and closed after 15 performances in February 1956.
Was one of the first actors to play the same character (Alexander Waverly) on two different television series: "The Man from U.N.C.L.E." (1964) and its spin-off, "The Girl from U.N.C.L.E." (1966) . Others actors who have done this are Gale Gordon, Frank Cady, Richard Anderson, Martin E. Brooks, Marla Gibbs, David Hasselhoff and Fred Dalton Thompson.