As a stage actor in the West End and on Broadway, he was associated with a wide range of works by modern playwrights, including Bernard Shaw, John Galsworthy and J.

[1] He began his acting career in the theatre in 1895, and learned his craft as a member of Willie Edouin's company, playing brash comic roles.

[1] His wife accompanied him, and when Gwenn was in a production of Ben Hur that was a disastrous failure, she restored the couple's fortunes by accepting an engagement from Williamson.

[3] Later, the couple appeared on stage together in London in a farce called What the Butler Saw in 1905[4] and, in 1911, when Irene Vanbrugh made her debut in variety, she chose Terry and Gwenn to join her in a short play specially written by J. M.

[5] When he returned to London, Gwenn appeared not in low comedy but in what The Times called "a notably intellectual and even sophisticated setting" at the Court Theatre under the management of J. E. Vedrenne and Harley Granville-Barker.

In Barrie's What Every Woman Knows (1908) in the role of the over-enthusiastic James Wylie he impressed the producer Charles Frohman, who engaged him for his repertory company at the Duke of York's Theatre.

A. Milne's The Great Broxoff (1923); Leo Swinburne in Good Luck by Seymour Hicks and Ian Hay (1923); and Hippolyte Gallipot in Lehár's Frasquita (1925).

[1] Of Gwenn's many British film roles, The Times considered his best known to be Jess Oakroyd in The Good Companions with John Gielgud and Jessie Matthews (1933) and Radfern in Carol Reed's Laburnum Grove with Cedric Hardwicke (1936).

[9] Gwenn appeared in more than eighty films, including Pride and Prejudice (1940), Cheers for Miss Bishop, Of Human Bondage and The Keys of the Kingdom.

Eventually, Gwenn bought a house at 617 North Bedford Drive in Beverly Hills, which he later shared with the former Olympic athlete Rodney Soher.

On March 5, 2023, Gwenn's misplaced urn was located in Vault 5 of Chapel of the Pines Crematory by researcher Jessica Wahl and Hollywood Graveyard YouTube channel creator Arthur Dark.