Jean Peters had never acted before this film. She replaced Linda Darnell, who was originally cast in the lead but shortly before production began 20th Century-Fox took her off the picture and gave her the lead in Forever Amber. Peters was given the lead in this picture based on a screen test she had taken at Fox just a week before filming began that had impressed the film's producers.
Alfred Newman's "Conquest" theme was adopted by the University of Southern California athletic teams.
Alfred Newman's score is one of the earliest soundtracks to be released as a separate album. It consisted of 18 minutes of music on 3 double-sided 78rpm records.
In several of the scenes in Mexico, a large smoke cloud is seen on the horizon. At the end of the movie smoke is shown rising from a volcano. This is most likely a fortuitous eruption of the Paricutin volcano in 1947, when the movie was filmed, standing in for the eruption of Popocatepetl on Cortez's day.
Initially Darryl F. Zanuck intended to release this film as a roadshow presentation with an intermission.
When Director of Photography Arthur E. Arling fell behind schedule, Charles G. Clarke was taken off of Miracle on 34th Street and replaced him.