This character is not a real person, and it is believed that Eastwood based the character on a combination of real-life outlaws. In the movie, William Munny is hired to bring in Billy the Kid, dead or alive. The book that inspired the script, "The Shootist," centered on a character that was inspired by John Wesley Hardin, one of the most deadly gunmen of the West. Munny was partly inspired by him.
There has been mention that Munny was partly based on Cullen Baker as well. Baker was a Tennessee-born Texas and Arkansas desperado whose gang is alleged to have killed hundreds of people, including former slaves during the early days of the American Old West.