He was born in 1912 in Manchester, England. He was educated at Shrewsbury School, and attended Brasenose College, Oxford. While at Oxford, he contributed film reviews to weekly undergraduate papers.

He began his career in 1936 as a film reviewer for several London newspapers.

During World War 2 he was stationed at Camp X in Canada. One of several training facilities operated by Special Operations Executive to train spies and special forces teams. He was the Political Warfare officer from 1942-44 and held the rank of Major. Dehn took part in missions in France and Norway.

He narrated the 1951 film Waters of Time and later wrote plays, operettas, and musicals for the stage. He wrote the lyrics for two films, The Innocents (1961) and Moulin Rouge (1952).

In 1949 or 1950, Dehn started a professional relationship with composer James Bernard, who later became his life partner. Dehn asked Bernard to collaborate with him on the original screen story for the Boulting Brothers film Seven Days to Noon (1950).

Through the 1960s, Dehn concentrated on espionage films, notably The Spy Who Came in from the Cold (1965), and The Deadly Affair (1967). He later wrote the Planet of the Apes sequels and the libretto for William Walton's opera The Bear.

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