Trivia

Jennifer O'Neill had some small roles in a couple of previous pictures. Director Hawks became fed up with what he perceived as her uncooperative nature and lack of experience: "She worked for about two days and then she thought she was so good she didn't have to work. [] She told me that she was a great horsewoman and when we fixed up the scenes of her riding, she looked like she was making a comedy"
--Source: Howard Hawks: interviews, Page 152

Director Howard Hawks: "And another thing, I'll tell you very frankly, I'm not very interested in making pictures about old men. [] Wayne had a hard time getting on and off his horse; he can't move like a big cat the way he used to. He has to hold his belly in; he's a different kind of person."
--Source: Hawks on Hawks, Page 118

Writer Leigh Brackett was brought onto the project after Hawks fired writer Burton Wohl. She followed the director's instructions to push the story back toward Rio Bravo and El Dorado. "Most of what I did on Rio Lobo was to try and patch over the holes. If these people ride into town and go into the saloon and shoot somebody — why? Nobody knows. And you try to figure out why. So that was mostly what I did"
--Source: Backstory 2: Interviews with Screenwriters of the 1940s and 1950s, Page 22

This was the fifth and last collaboration over 22 years period between John Wayne and director Howard Hawks.Red River (1948) Rio Bravo (1959) Hatari! (1962) El Dorado (1966) and Rio Lobo (1970)

Jack Elam, playing the Old Man, was more than a decade younger than John Wayne.

During a break in filming John Wayne collected his Best Actor Oscar for True Grit at The 42nd Annual Academy Awards. When he returned, every member of the cast was wearing an eye patch.