Harrison turned professional the following year after signing a contract with BBC Sport, and went on to have seventeen fights on the network before their cancellation of all boxing broadcasts.
He retained the title in 1998, defeating Dean Redmond,[5] and won gold at the 1998 Commonwealth Games beating Michael Macquae of Mauritius in the final.
[6] His debut was against US club fighter Michael Middleton, whom Harrison knocked out in the first round in Wembley Arena with 6 million viewers watching at home.
[8] Harrison continued to win and made his United States debut in November 2002, knocking out Shawn Robinson in the 1st round.
In the 2003 November issue of the Ring Magazine, Harrison was tipped to emulate Lennox Lewis and become a dominant world champion.
Harrison returned to the UK in December to face long-time bitter rival Danny Williams in London for the Commonwealth title.
Harrison claimed that he could make a comeback, but Warren suggested that any return to the ring would be for a reduced purse, since the public would have no great interest.
However, he and his coach Kelvyn Travis were involved in a car accident on 21 September 2007 in the United States, and Harrison suffered injuries that caused the fight to be cancelled.
[28] Harrison underwent surgery for his injuries,[29] and returned on 19 April 2008, beating the American Jason Barnett in the fifth round on the undercard of the Bernard Hopkins vs. Joe Calzaghe fight at the Thomas & Mack Center in Las Vegas, Nevada.
After proposed matches with Samuel Peter and Martin Rogan fell through,[31] on 6 September 2008 Harrison gained what the BBC described as "an unconvincing victory" over George Arias at the MEN Arena in Manchester.
On 6 December 2008, Harrison's career seemed all but over as he was defeated by the winner of the first Prizefighter tournament and Belfast taxi driver, Martin Rogan.
Following his success in the Prizefighter tournament, it was announced on 15 January 2010 that Harrison would face Albert Sosnowski for the European Boxing Union heavyweight title, with the fight set for 9 April 2010.
BBC Sport said of the fight: "Having come within seconds of a defeat that would have made a mockery of pre-fight assertions that he could face one of the Klitschko brothers for a world title, Harrison said: 'I had to win it somehow.
'"[35] On 24 April 2010, Harrison underwent surgery in Cheadle, Greater Manchester to repair the torn Pectoralis major muscle.
[40] Due to the nature of Harrison's defeat, the BBBofC withheld some of the fighter's purse while a full investigation of the bout was carried out.
[44] Former British and Commonwealth champion Derek Chisora was critical of the decision, stating, "He's going to box on, but who's going to buy the tickets to go and watch him?
"[45] On 15 November 2011, during an interview on BBC Breakfast following his departure from Strictly Come Dancing, Harrison announced that he would return to boxing for one last time saying "It could be over, but I just need to go and check."
Harrison returned to the ring on 26 May 2012 and boxed Ali Adams at the Brentwood Centre, Essex for the International Masters Championships.
[48] On 23 February 2013 Harrison won the Prizefighter 29: The International Heavyweights III tournament, defeating Derric Rossy in the final.
[49] On 27 April 2013, Harrison stepped into the ring to fight the unbeaten American prospect Deontay Wilder, who had a record of 27 wins, all knockouts [27–0–0].
[56] In 2015 Harrison revealed he was suffering from traumatic brain injuries leading to sight problems and severe mood swings.