Born in Evanston, Illinois. A Harvard graduate, he worked for newspapers in Boston and New York and in advertising before being brought out to Hollywood by Ben Hecht. He wrote screenplays, adaptations and original stories, alone or in collaboration, for many successful films, mainly at MGM, where he was a favorite of director Victor Fleming. He also doctored many other scripts without screen credits. Some of Mahin's notable writing credits (alone or in collaboration) include SCARFACE (continutity & dialogue) and RED DUST (both 1932), THE PRIZEFIGHTER AND THE LADY, BOMBSHELL and ESKIMO (all 1933), TREASURE ISLAND and CHAINED (both 1934), NAUGHTY MARIETTA (1935), RIFFRAFF, SMALL TOWN GIRL and THE DEVIL IS A SISSY (all 1936), A STAR IS BORN (uncredited contributing writer), CAPTAINS COURAGEOUS and THE LAST GANGSTER (both 1937), TEST PILOT (1938, uncredited), THE WIZARD OF OZ (1939, uncredited), FOREIGN CORRESPONDENT (uncredited) and BOOM TOWN (both 1940), DR. JEKYLL AND MR. HYDE and JOHNNY EAGER (both 1941), WOMAN OF THE YEAR (uncredited) and TORTILLA FLAT (both 1942), THE YEARLING (1946, uncredited), QUO VADIS? and SHOW BOAT (both 1951), MY SON JOHN (1952), MOGAMBO (1953), ELEPHANT WALK (1954), THE BAD SEED (1956), HEAVEN KNOWS, MR. ALLISON (1957) and NO TIME FOR SERGEANTS (1958). In 1958 he started the Mabin-Rackin production company with screenwriter Martin Rackin. His writing credits there include THE HORSE SOLDIERS (1959, also uncredited producer), THE BARBARIANS and NORTH TO ALASKA (both 1960, both also producer), THE SPIRAL ROAD (1962) and MOMENT TO MOMENT (1965). Mahin was among the founders of the Screen Writers Guild in 1933, but later in the decade helped found and became president of the competing Screen Playwrights, a company union. During the Red Scare of the late 40s and early 50s, he was associated with conservative groups that encouraged the weeding out of purported Communist influences in Hollywood. At one time he was married to actress Patsy Ruth Miller.  Nominated for Writing (Screenplay) 1937: CAPTAINS COURAGEOUS (w. Marc Connelly & Dale Van Every)  Nominated for Writing (Best Screenply based on material from another medium) 1957: HEAVEN KNOWS, MR. ALLISON (w. John Huston) 2 nominations