She began her career as a Dancer in West End music halls, and at the age of 17 she began going out with Pownoll Pellew (later 9th Viscount Exmouth), and they shared an interest in Sports cars. She made her film debut in How's Chances? (1934) in a small part, and had a larger role in Get Your Man, another 1934 film. She continued to act in "quota quickies" films for several years. Walsh first met David Lean, then a film Editor, in 1936, during the filming of Secret of Stamboul. They began a relationship, and Walsh broke her engagement to Pellew. Walsh and Lean married on 23 November 1940. She moved on to prestige films with appearances in In Which We Serve (1942) and This Happy Breed (1944), both directed by Lean and written by Noël Coward. Walsh campaigned for Lean to receive co-director credit on In Which We Serve.