Robert Mitchum Sings the Title Song
It takes awhile to recognize his voice -- the famous bullfrog baritone has to go high for some notes -- but damn, that's HIM.
In the opening credits the song is billed, but the singer isn't. And you wonder.
But at the very, very end of the movie, at the end of the (brief) end credits, Mitchum gets a great big singing credit.
IMDb claims that Mitchum wanted to sing the song because Dean Martin sung the theme song to the Martin-Mitchum Western Five Card Stud one year before.