Her theatre work led to appearances on radio and phonograph records as well as a supporting role in the film Barnacle Bill (1935).

Her American film debut came as the lead in Little Big Shot (1935), directed by Michael Curtiz and co-starring Glenda Farrell, Robert Armstrong, and Edward Everett Horton.

Jason followed this with supporting roles opposite some of Warner Bros. most popular stars, including Kay Francis in I Found Stella Parish (1935), Al Jolson in The Singing Kid (1936), Pat O'Brien and Humphrey Bogart in The Great O'Malley (1937), and again with Kay Francis in Comet Over Broadway (1938).

Warners also starred her in The Captain's Kid (1937), and four Vitaphone two-reelers filmed in Technicolor: Changing of the Guard, A Day at Santa Anita, Little Pioneer, and The Littlest Diplomat.

Jason never became the major rival to Shirley Temple that Warner Bros. had hoped, and her film career ended after playing two supporting roles at 20th-Century Fox.