This has been described as the first "spaghetti western", but when this film was made, there had already been about 25 such westerns produced in Italy. This was, however, the first to receive a major international release.
This was the first time that Sergio Leone and composer Ennio Morricone worked together. Initially Leone was not keen on using Morricone for this film. Lacerenza's initial trumpet performance of the score made Leone quickly set aside any reservations. Leone and Morricone, who had known each other since 3rd grade, would develop a close working relationship that would last through all of Leone's future films.
When Clint Eastwood arrived on the set, he was struck by how little the Italian crew and writers knew about the American West they were filming about. For example, he had to point out that coonskin caps were worn by frontiersmen and trappers in the Davy Crockett era, circa the 1820s, not by gunfighters and townsmen in the American West and Mexico of the 1870s, as the scriptwriters had written.
When the film made its U.S. network television debut on "The ABC Sunday Night Movie" in February 1975, a new prologue was added in which an unidentified lawman or politician (played by Harry Dean Stanton) orders "Joe" to get rid of the gangs of San Miguel in return for a pardon. Neither Eastwood nor Leone were involved in the shooting of this additional footage. A double with his face hidden and stock footage of Eastwood were used. Monte Hellman directed the new footage. This prologue is now available on the Special Edition released in 2005.