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.
Robert Redford once said of doing his own stunts on the film: "I like the tough stuff. Half the fun of making movies is doing the action scenes. Anyone can say words. Don't get me wrong. The stunt guys are really necessary and I never do the stunts where a pro can pull it off safer and better. But I do like to do the action where the camera is too close to tell a lie and the movie's insurance men are back at the office making out policies."
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, who was playing Jeremiah Johnson's Indian wife Swan.