While still a student at Cambridge, Belfrage began his writing career as a film critic, publishing his first article in "Kinematograph Weekly" in 1924. In 1927 he went to Hollywood, where he was hired by the "New York Sun" and "Film Weekly" as a correspondent. Belfrage returned to London in 1930 as Sam Goldwyn's press agent. Returning to Hollywood, he became politically active, joining the Hollywood Anti-Nazi League and co-editing a left-wing literary magazine called "The Clipper". He decided to make the United States his home and took out first papers for citizenship in 1937, although he failed to complete the process within the statutory seven-year time limit.