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).
The Italian release title of the film was called "Red Crow You Will Not Have My Scalp".
Trapper John Johnston's body was buried in the Veteran's Cemetery in Los Angeles. After the movie came out, Johnston's body was reburied at Old Trail Town in Cody, Wyoming. Robert Redford was a pallbearer in the reburial ceremony attended by 2,000 people.
"Fort Hawley" mentioned twice in the movie was the actual historical Fort Hall, a well-known trading post established in 1834 along the Snake River in present-day southern Idaho.
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 Blackfeet 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.
Liver Eatin' Johnston's wife (who was pregnant at the time) was actually killed by a random raiding party of Blackfeet not in revenge for a violation of their burial grounds. She was killed in the Spring while Johnston was off trapping and he didn't return to find her body until several months later. He identified the band that had killed her because he recognized a Tennessee rifle he had given her in the possession of a Crow warrior. Also, rather than isolated incidents as shown in the movie, Johnston often recruited other mountain men as well as Indians (particularly Flatheads) to help him with his vendetta. The part about the warriors sent to kill him and told not to return without his scalp was true.