Based upon a real-life trapper named John Johnston, nicknamed "Crow Killer" and "Liver Eater Johnston" for his penchant for cutting out and eating the livers of Crow Indians he had killed (several Crows had murdered his wife and he swore vengeance against the entire tribe).
Trapper John Johnston's body was buried in the Veterans Administration cemetery in Los Angeles, CA. After the movie came out, Johnston's body was reburied at Old Trail Town in Cody, WY. Robert Redford was a pallbearer in the reburial ceremony attended by 2,000 people.
Scores of American Indians from northern Utah were hired as actors, extras and background artists.
The movie's technical director was a Flathead American Indian from Montana. One of his key jobs was to act as coach for Delle Bolton (Swan), on how to act as a chief's daughter and a mountain man's wife.
Robert Redford has described this as his favorite of all his movies. He likes that Johnson suffers but he continues on.