de Marney's career in the theatre began in 1923 and continued almost without interruption, taking in film, radio and television parts. He toured with Mrs. Patrick Campbell in The Last of Mrs. Cheyne. In 1930 he played Gustave in The Lady of the Camellias, and toured South Africa as Raleigh in Journey's End. In 1934 he played Tybalt in Romeo and Juliet at the Open Air Theatre, and Giovanni in 'Tis Pity She's a Whore at the Arts.
Thrillers tended to be his stock in trade, appearing in a revival of Sutton Vane's Outward Bound during the 1930s, as well as Agatha Christie's Ten Little Indians, Dear Murderer, as well as a revival of Gerald Du Maurier's Trilby in later years.
He also appeared on radio as the Count of Monte Cristo, and was the first actor to portray Leslie Charteris' Simon Templar on radio, when The Saint debuted on Radio Athlone in 1940 for six episodes.
In 1931 he became director of the Connaught Theatre, Worthing, and in 1932, with his brother, the actor Derrick De Marney, he founded the Independent Theatre Club at the Kingsway Theatre, where he directed Emil Ludwig's Versailles and an adaptation of Schnitzler's novel Fraulein Else. He also directed Louis Golding's Magnolia Street Story and Master Crook, originally called Cosh Boy. With his brother he alternated as Slim Callaghan in Meet Slim Callaghan at the Garrick Theatre and carried on the same role in the play's sequel Slim Carves, which he produced and directed.
He also made his film debut in 1931, and went on to appear in a number of quota quickies of the period, including mystery horror films The Unholy Quest (1934) and The Mystery of the Marie Celeste (1935), the latter opposite Bela Lugosi. These roles in the macabre would continue throughout his career and took in films such as The Pharaoh's Curse (1957), Boris Karloff vehicle Die, Monster, Die (1965) and The Hand of Night (1966).
After starring in 'B' movies like Duel Alibi (1948), and No Way Back (1949), he uprooted to Hollywood, where he appeared in a number of famous television series such as Bonanza, Wagon Train, Maverick, and The Twilight Zone. He was a series regular on Johnny Ringo. As well as small roles in films such as The Ten Commandments (1956), and Spartacus (1960). He returned to Britain in the 1960s and continued to appear in television series such as Maigret, Dr. Finlay's Casebook, Doctor Who, and Z-Cars. His last film appearance was in The Strange Affair (1968).
He died in 1971 following an accident on the London Underground.