Veteran Williams, who knocked out Mike Tyson in a career-best win in 2004, looked every inch of his 45 years as McAllister took it to him from the off.
Despite a four-stone weight differential, McAllister had Williams down three times in the first half of the fight. He finished the job in the tenth at the Beach Ballroom in Aberdeen.
Having seen enough and with Williams faded, the one-time British and Commonwealth ruler was pulled out to save him further punishment.
McAllister had most recently been campaigning as a welterweight. He made the jump up when Williams accepted a tongue-in-cheek offer to fight.
The 35-year-old had only had one previous outing above 147 pounds. McAllister managed to amaze his home fans by taking down the much bigger Williams.
Williams was also on an unbeaten streak. The heavyweight had three bouts already lined up for later this year. He announced his immediate retirement after the fight.
The shock win could see McAllister gain another high-profile chance in the future as ‘The Aberdeen Assassin’ bids to enhance his reputation further.
Without a fully sanctioned fight in five years, though, McAllister could find it hard to nail down the bigger names in the UK.
