After the war, Todd was unsure what direction to take in his career. His former agent, Robert Lennard, had become a casting agent for Associated British Picture Corporation and advised him to try out for the Dundee Repertory Company. Todd did so, performing in plays such as Claudia, where he appeared with Catherine Grant-Bogle, who became his first wife. Lennard arranged for a screen test and Associated British offered him a long-term contract in 1948. He was cast in the lead in For Them That Trespass (1949), directed by Alberto Cavalcanti. The film was a minor hit and Todd's career was launched.
Todd had appeared in the Dundee Repertory stage version of John Patrick's play The Hasty Heart, portraying the role of Yank and was chosen to appear in the 1948 London stage version of the play, this time in the leading role of Cpl. Lachlan McLachlan. This led to his being cast in that role in the Warner Bros. film adaptation of the play, filmed in Britain, alongside Ronald Reagan and Patricia Neal. Todd was nominated for the Academy Award for Best Actor for the role in 1949. He was also voted favourite British male film star in Britain's National Film Awards. The film was the tenth most popular movie at the British box office in 1949.