Cooper was born as Frank James Cooper, and changed his name to Gary in 1925, following the advice of his agent.
Cooper was born in Helena, Montana, but lived in England with his mother and attended school there for seven years. When he was thirteen years old he was injured in an automobile accident, and had to move to his father's cattle ranch in Montana to recuperate, which is where he gained his riding skills. In 1924 he moved to Los Angeles with the intention of becoming a commercial artist, but was not very successful. After three months he became an extra in the motion picture industry. A year later he had a chance at a real part in a two reeler with Eileen Sedgewick as his leading lady. After the release of this short film he was called to Paramount Studios and offered a long-term contract, which he accepted.
He appeared in over a hundred films, including:
Children of Divorce (1927), Wings (1927), Arizona Bound (1927), Nevada (1927), Beau Sabreur (1928), The Legion of the Condemned (1928), Half a Bride (1928), The First Kiss (1928), The Shopworn Angel (1928), The Wolf Song (1929), Betrayal (1929), The Virginian (1929), Seven Days Leave (1930), Only the Brave (1930), The Texan (1930), A Farewell to Arms (1932), Alice in Wonderland (1933), The Lives of a Bengal Lancer (1935), Mr. Deeds Goes to Town (1936)(Nominated), Beau Geste (1939), Meet John Doe (1941), Sergeant York (1941)(Won), The Pride of the Yankees (1942)(Nominated), For Whom the Bell Tolls (1943)(Nominated), Saratoga Trunk (1945), The Fountainhead (1949), Distant Drums (1951), and High Noon (1952)(Won).