Distinguished by his relaxed charm and smooth, "black velvet" voice,[1] he had a lengthy career in theatre, film, television and radio, often playing aristocratic or gentlemanly, sophisticated roles.

[1] Born to a Jewish family in London, Genn was educated as a lawyer and was a practising barrister until after World War II, in which he had served in the Royal Artillery as a lieutenant-colonel.

[2] Genn attended the City of London School, having gained scholarships in both classics and mathematics,[3] and studied law at St Catharine's College, Cambridge, where he became captain of both the football and tennis teams.

[5] Genn entered acting with the Berkley Players (attached to the West London Synagogue)[4] in order to increase his chances of finding prospective clients for his legal work.

In 1938, Genn appeared in the theatrical hit The Flashing Stream by Charles Langbridge Morgan and went with the show to Broadway in New York City.

His many other stage performances included Lillian Hellman's Another Part of the Forest, 12 Angry Men, The Devil's Advocate, and Somerset Maugham's The Sacred Flame.

In 1974, a recording of The Jungle Book was released with Genn as narrator and Miklós Rózsa conducting the Frankenland Symphony Orchestra with the music from the film.

Genn received another small role in Alexander Korda's The Drum (1938) and was the young man who danced with Eliza Doolittle at the duchess's ball in Pygmalion, a film made in the same year, although he was uncredited.

In 1944 the actor was given official leave to appear as Charles I d'Albret, the Constable of France, in Laurence Olivier's Henry V. Genn was awarded the Croix de Guerre in 1945.