Most of this movie was shot in Eckley, Pennsylvania. "Paramount Pictures" saved the town from being destroyed. It was slated to be demolished for strip mining, but after the movie was filmed, the town's land was donated to the Pennsylvania Historical and Museum Commission. The town is now a museum. Several structures built for the movie still survive.
During filming, Sir Sean Connery and Richard Harris became good friends. Harris later agreed to play the cameo role of King Richard the Lionheart in "Robin and Marian (1976)" as a favor to Connery, and in the late 1980s, there was talk of them making another movie together, but this never happened.
The real Jack Kehoe was not a miner, but the owner of a saloon, The Hibernian House. His descendants run it to this day.
According to cinematographer James Wong Howe, director Martin Ritt wanted to shoot this movie in black-and-white, but was not allowed to do so by Paramount Pictures. By 1970, studios were concerned that black-and-white movies would not make as much money when broadcast on television, possibly for fear that they would be considered "old" movies.
Jack Kehoe was pardoned in 1979, by Governor Milton Shapp.