Johnny Cash first recorded one of his most famous songs, "Folsom Prison Blues" in 1955. While he was in the Air Force he saw a documentary called, Inside The Walls of Folsom Prison (1951). At about this time he also heard Gordon Jenkins' song "Crescent City Blues." He put both of these experiences together and wrote "Folsom Prison Blues." According to Anecdotage.com, he had this to say about the song's most infamous line ("I shot a man in Reno just to watch him die"): "I sat with my pen in my hand, trying to think up the worst reason a person could have for killing another person, and that's what came to mind."
In the late Sixties, when Cash's career was stalled, he suggested recording a live concert album at a prison. He contacted San Quentin and Folsom, both in California. Folsom agreed to host him, and his entourage, which included his band, the Statler Brothers, Carl Perkins, and June Carter Cash arrived at Folsom on January 13, 1968. They had two concerts, at 9:40 AM and then again at 12:40 PM. They had scheduled two in case the first did not have enough good quality recordings.
The rest, as they say, is history. The morning concert was good enough to nearly fill the album AT FOLSOM PRISON, which was released that spring. It eventually became multi-platinum, with more than three million units sold. The single of a live recording of "Folsom Prison Blues," hit Top Forty on the Pop chart and Number One on the Country chart in the summer of 1968.
Johnny Cash eventually paid Gordon Jenkins a settlement for the similarities between their two songs.
Here's a video of Johnny Cash playing in front of a prison audience. It isn't Folsom (that concert was not taped, just recorded) but it gives you an idea of what it must have been like at the time.