- John Wayne starred in Rio Bravo (1959) (of which this film is a remake), and after reading the script for "El Dorado" he asked to play J.P. Hara, but the part went to Robert Mitchum.
- The opening credits feature a montage of original paintings that depict various scenes of cowboy life in the Old West. The artist was Olaf Wieghorst, who appears in the film as the Gunsmith, Swede Larsen.
- The poem recited by Mississippi is an actual poem called "El Dorado" by Edgar Allan Poe.
- Robert Mitchum revealed in an interview that when Howard Hawks asked him to be in the film, Mitchum asked what was the story of the film. Hawks reportedly replied that the story didn't matter because the film had some "great characters".
- Robert Mitchum's character was wounded and needed to use a crutch, but Mitchum would switch which arm he used with the crutch through out shooting. The continuity was so poor that Wayne (who actually worked continuity in silents while a star college football player, a method used by Hollywood fans to slip players some spending money) had his character mention it in one of the last scenes. The director enjoyed it so much he left it in the movie.
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