Haycox was born in Portland, Oregon, to William James Haycox and the former Martha Burghardt on October 1, 1899. After receiving an education in the local schools of both Washington state and Oregon, he enlisted in the United States Army in 1915 and was stationed along the Mexican border in 1916. During World War I he was in Europe, and after the war he spent one year at Reed College in Portland. In 1923, Haycox graduated from the University of Oregon with a Bachelor of Arts degree in journalism, Five join SOJC's Hall of Achievement where he also started writing under professor W. F. G. Thatcher. In 1925, Haycox married Jill M. Chord, and they would have two children.
He published two dozen novels and about 300 short stories, many of which appeared first in pulp magazines in the early 1920s. During the 1930s and 40s, he was a regular contributor to Collier's Weekly and The Saturday Evening Post. Fans of his work included Gertrude Stein and Ernest Hemingway, and the latter once wrote, "I read The Saturday Evening Post whenever it has a serial by Ernest Haycox."
His story "Stage to Lordsburg" (1937) was made into the movie Stagecoach (1939), directed by John Ford and featuring John Wayne in the role that made him a star. The novel Trouble Shooter (1936), originally serialized in Collier's, was the basis for the movie Union Pacific (1939), directed by Cecil B. DeMille, starring Barbara Stanwyck and Joel McCrea. Haycox wrote the screenplay for Montana (1950), directed by Ray Enright, which stars Alexis Smith and Errol Flynn.
Haycox died in 1950, at the age of 51, in Portland. In 2005 the Western Writers of America voted Haycox one of the 24 best Western authors of the Twentieth Century.