Interestingly, the title of this movie promotes a common misconception that was proved untrue in 1986. Two almost identical tintypes of Billy the Kid were taken at the same time in 1880. The original of one tintype disappeared years ago. The second original tintype was preserved for years in the Sam Diedrick family and came to light only in 1986. Since tintypes are reversed images, the picture from the first tintype led to the myth of the left-handed gun. After the second tintype came to light, the reversed image was reversed to show the Kid as he actually posed, with a Winchester carbine in the left hand and his holstered Colt single-action on his right hip. See Utley, Robert M., Billy the Kid, A Short and Violent Life, University of Nebraska Press, 1989. Statement following page 110 alongside the picture of Billy the Kid.
The character of "Tunstall" was based on a real person, John Henry Tunstall, an English immigrant who was murdered in 1878, aged 24 or 25 at time of death, under circumstances very similar to those portrayed in this film. However, in this, and every other film in which there's a portrayal of "Tunstall" [this film], "Henry Tunstall" [Chisum (1970)], or "John Tunstall" [Young Guns (1988)], not only is a different version of Tunstall's name used, but the actors portraying the Tunstall character have been double (or more) the age of the real life Tunstall at the time of his death. At the time of production of this film, Colin Keith-Johnston was approximately 61 years old; at the time of production of Chisum, Patric Knowles was approximately 58 years old; and at the time of production of Young Guns, Terence Stamp was approximately 49 years old.
Gore Vidal greatly disliked this well-received film version of his television play, "The Death Of Billy The Kid", once describing it as "a film only someone French could like." He was greatly annoyed when director Arthur Penn expressed criticisms of his original script and brought in Leslie Stevens for rewrites. In 1990, the TV movie "Billy The Kid" was made, not only as a remake of this film, but as a rebuttal of it, written and largely controlled by Vidal himself. He declared himself pleased with it, but the 1958 film remains better-known.
The sets for the town of Medaro were actually left over from Warners' 1939 epic Juarez (1939). Although they were in state of great disrepair, parts of them were salvaged for use in "The Left-Handed Gun."