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).
Many of the locations for the film were shot on or near Robert Redford's property in Utah (he owned approximately 600 acres there at the time), although some locations were as much as 600 miles away.
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.
According to the book "Crow Killer," the Crazy Woman was a real person who had settled in the Wolf Tail Valley. After her children were killed and her husband taken captive, she remained in her cabin. Liver Eatin' Johnson, Del Gue and Anton Sepulveda were among the mountain men who "avenged" her. One popular story was that the mountain man known as "Hatchet Jack" was actually her husband who had gone insane after being scalped and tortured by the Blackfoot Indians when they took him away. It was known that Hatchet Jack had been scalped at some point in his life and that he was mentally unbalanced. Johnson refers to this when he tells the Crazy Woman that he cannot find any sign of her husband, but that he might return if he escaped from the Indians.
Robert Redford has described this as his favorite of all his movies. He likes that Johnson suffers but he continues on.