Sheriff Bartlett's Journal contains an article from the newspaper "The Londoner" which tells Jack's story before the events of the game. He was born in Sandhurst, England (as he says in his multiplayer selection quote) and was raised as an Englishman. He mentions that he played rugby as a young man. However, he became a bank robber and after robbing a London bank, escaped minutes before a hanging party on a boat for America.
As a sharpshooter, he gained fame in the United States and was contacted by ringmaster Preston O'Leary, who asked Jack to join his circus as a tricks sharpshooter. Jack accepted, but before joining O'Leary was killed in an "accidental" fire, and Professor Perry took control.