Liston, Lyle, Qawi, Scott, Cummings, Hopkins... I'm also talking state prison, so no including guys like Patterson.
Amnat Ruenroeng started boxing while in prison. He then became succesful Olympic boxer - and eventually won professional world title at 35.
Does during prison count? i.e. James Scott? Didn't Adonis Stevenson serve time for being a pimp's "enforcer" & beating up prostitutes?
Anthony Joshua was in Reading Prison John Gulley back in the early 1800s Paul Sykes. He lost a Commonwealth title match to John Gardner. Gardner ended up losing to Michael Dokes not long before Dokes became champion. Clifford Etienne learned to box in prison. There was a documentary about prison boxing called Fight to the Max and he had a big part in it. Unfortunately he’s back in prison and will spend the rest of his life there. From that great late 70s-early 80s LHW era: James Scott was locked up in prison and learned to box and become a pro. Then he got locked up again for murder and had pro fights in Rahway against legit world class contenders and future champs like Dwight Qawi and Eddie Mustafa Muhammad. Obviously Qawi was an example. Eddie did time before he was a pro too, but I think it was county jail and not prison. I think Matthew Saad Muhammad was locked up in juvenile facilities, but not prison as an adult. Art Tucker was also locked in Rahway and had a couple fights there. He eventually lost to a young Riddick Bowe, as well as Tommy Morrison and Tim Witherspoon.
Eddie Machen "He was convicted of armed robbery in 1952 and served three years in prison. It was not until he turned professional in 1955 that Machen laced on another glove in recognized competition".