During World War I, he served in the military[1] and was afterwards appointed assistant national director of the American Jewish Relief Committee.
He later became a drama and film critic for the Jewish Tribune until the early 1920s, when he went to Hollywood to become a reader for Samuel Goldwyn.
Later he worked as a script clerk for directors King Vidor and Victor Sjöström before becoming a screenwriter at MGM in 1924.
Lewin was appointed head of the studio's script department and by the late 1920s was Irving Thalberg's personal assistant and closest associate.
He was the best friend of actress Devi Dja and cast her in three of his movies but failed to get her a leading role in The Moon and Sixpence (1942).