Thesiger first came to public notice in the farce A Little Bit of Fluff by Walter W. Ellis at the Criterion Theatre in 1915-18. He played the comic hero Bertrand Tully over 1,200 times, and received very favourable reviews. The London Standard wrote, “The character of Bertram Tully...is one of the happiest seen on the stage for a decade past...Mr. Ernest Thesiger achieved a real triumph.” He reprised the role in the 1919 film version, and in a 1923 revival at the Ambassadors Theatre.
When he appeared in a Christmas production of The Merry Wives of Windsor in 1919, Thesiger met and befriended James Whale. After Whale moved to Hollywood and had found success with the films Journey's End (1930) and Frankenstein (1931), the director was commissioned to direct the screen adaptation of J. B. Priestley's Benighted as The Old Dark House (1932), starring Charles Laughton in his first American film, together with Boris Karloff and Raymond Massey. Whale immediately cast Thesiger in the film as Horace Femm, launching his Hollywood career. The following year Thesiger appeared (as a Scottish butler) with Karloff in a British film The Ghoul.