According to an article about Steve McQueen's western films in Cowboys & Indians Magazine (to celebrate the DVD release of Wanted: Dead or Alive (1958)), McQueen thoroughly researched the life of Tom Horn by spending time with famed western novelist Louis L'Amour, who had many of Horn's letters in his private collection.
The type of gallows used to hang Horn from are called "Julian Gallows". This device uses water to initiate the action. Not as efficient as the more conventional method, reportedly the first use of the contraption (not for Horn) required a wait of 30 minutes before the trap dropped.
When the film was released some critics suggested that Steve McQueen had lost his charismatic screen presence after being away for too long. He had retired from acting after The Towering Inferno (1974), returning three years later to make An Enemy of the People (1978) which was never properly released.
Tom Horn's rifle (a Winchester 1894 in .30-30) was also used to put down the famous bronco Steamboat (the basis for the 'bucking horse and rider' on Wyoming license plates) after he was injured at a rodeo in Salt Lake City in 1914.
William Goldman wrote an early version of the screenplay, with Don Siegel attached as director. Siegel dropped out after disagreements with Steve McQueen (whom Goldman found to be extremely eccentric), and Goldman's script, which McQueen rejected, was later the basis for Mr. Horn (1979). This was rushed into production to compete with this film, making it onto the airways several months ahead of the film's much-delayed release.
According to director Arthur Penn, the real-life Tom Horn was the inspiration for the villainous Marlon Brando character in his film The Missouri Breaks (1976). In interviews, Penn described his screenwriter, Thomas McGuane, as an expert on the life of Tom Horn, which may be why McGuane is also one of the screenwriters on this rather different version of Horn's adventures.
The rifle Horn uses to such deadly effect in the film is an original Winchester Model 1876 in .45-60 caliber, fitted with a custom tang sight. Manufactured from 1877-1894, the Model '76 was an obsolete arm by the turn of the century, when the events of the story take place. All the available historical sources state that Horn actually used a .30-30 Winchester Model 1894 for his controversial activities as a "stock detective" in Wyoming. Horn gave this rifle to rancher C.B. Irwin not long before Horn's execution on November 20, 1903. It resides today in the collection of the National Cowboy Hall of Fame in Oklahoma City.
Post-production was fraught - the producers attempting both a linear version of the film and then another telling the story in flashback, before settling on the former approach. The film was still being reedited ahead of its March 1980 release date, but to no avail - it received poor reviews and was another box office failure.
Steve McQueen initially wanted to adapt Harold Pinter's play Old Times but First Artists insisted that he instead film this, a script they had owned for some time, as the final film in the star's three picture deal he had signed with them under Warner Bros.