Seaton, along with fellow Writer and friend Robert Pirosh, joined Metro-Goldwyn-Mayer as a contract Writer in 1933. His first major screen credit was the Marx Brothers comedy A Day at the Races in 1937. In the early 1940s, he joined 20th Century Fox, where he remained for the rest of the decade, writing scripts for Moon Over Miami, Coney Island, Charley's Aunt, The Song of Bernadette, and others before making his directorial debut with Diamond Horseshoe in 1945. From this point on he was credited as both Screenwriter and Director for most of his films, including The Shocking Miss Pilgrim, Miracle on 34th Street, Apartment for Peggy, Chicken Every Sunday, The Big Lift, For Heaven's Sake, Little Boy Lost, The Country Girl, and The Proud and Profane.