William Harrison Dempsey was Jack Dempsey real name. He was born June 24, 1895 in Manassa Colorado and past away May 31, 1983 due to a heart failure.. He was the World Heavyweight Champion from 1919 to 1926. He is #7 on the top 100 boxers ever. He is part of the International Boxing Hall of Fame and still today is to be one of the hardest punchers ever. That's why he got the nickname Manassa Mauler.