Jack qualified for the 2008 Olympics for Gambia in the middleweight division, and was the nation's flag bearer at the Opening Ceremonies.
[10] Jack started his professional career in his native Sweden where he won his debut fight by unanimous decision on 6 June 2009.
Including defeating Adam Collins by TKO on 16 December 2011[12] and a unanimous decision over 6 rounds (60–54, 59–55 and 58–56) over Grover Young on 7 March 2012.
[14][15] Following his success extending his record to 11–0, Jack was spotted by Floyd Mayweather in a sparring session in opposite Andre Dirrell.
[21] In his first title fight, Jack challenged Marco Antonio Periban for the NABF super middleweight championship on 12 September 2013.
[27] Jack got back into the win column on 30 August 2014 when he fought Jason Escalera at the Palms Casino and Resort, Pearl Theater in Las Vegas, Nevada,[28] and won by unanimous decision 100–90, 100–90, and 99–91.
[29] Jack's next fight was due to take place on 12 December 2014 against Mexican boxer Francisco Sierra at the Alamodome in San Antonio, Texas on the undercard of Erislandy Lara vs. Ishe Smith.
[31][32] On 17 February 2015 it was announced that Jack would be challenging undefeated Anthony Dirrell (27–0–1, 22 KOs) for his WBC super middleweight championship.
"[38][39][40] On 17 June 2015 it was announced that Jack would make his first defence against mandatory challenger George Groves (21–2, 16 KOs) on 22 August in Las Vegas.
[42] On 21 July, the fight was pushed back to instead take place on the undercard of Floyd Mayweather Jr. vs. Andre Berto on 12 September at the MGM Grand Garden Arena.
[59] Showtime Boxing confirmed that a unification fight between Jack and IBF world champion James DeGale (23–1, 14 KOs) was set for 14 January 2017.
Post-fight Jack's promoter Floyd Mayweather Jr, complained about the result saying his fighter had done enough to win and calling the decision "bad for boxing."
[62][63][64] On 16 January 2017, the WBC ordered negotiations to begin for Jack to make a mandatory defence against British boxer Callum Smith (22–0, 17 KOs).
[68] On 19 June, Jack revealed that his team were working towards a deal for him to fight WBA (Regular) champion Nathan Cleverly.
He expected when the fight gets announced, it would be on the undercard of Floyd Mayweather Jr. vs. Conor McGregor in Las Vegas on 26 August 2017.
It was also reported that whether the WBA (Regular) title would be at state at this point was uncertain, as mandatory challenger Bivol was yet to agree a step aside fee.
[73] On 12 August, the WBA's Championships Committee granted special permission for the fight to be contested for their light heavyweight title.
"[80] On 8 September 2017 promoter Yvon Michel disclosed that there was serious ongoing negotiations between Stevenson and mandatory challenger Eleider Álvarez (23-0, 11 KOs) to fight before the end on 2017.
The WBC went on to state they would order Álvarez to fight Ukrainian boxer Oleksandr Gvozdyk (14–0, 12 KOs) for the interim title.
Yvon Michel admitted he was working on a deal to keep Álvarez happy and this was the same reason as to why tickets had not yet gone on sale for the Stevenson-Jack fight, which was a month away.
[92] On 18 April, it was announced that a deal had been reached for Álvarez to challenge Sergey Kovalev (32–2–1, 28 KOs) for the WBO light heavyweight title on HBO.
[101] In November 2018, speaking to Swedish news agency TT, Jack revealed he would not fight again in the year and would likely return in early January 2019.
[102] Light heavyweight upcomer Marcus Browne (22–0, 16 KOs) called out Jack to fight him in what was an 'open division' as both traded words on Instagram.
[103][104][105] According to RingTV.com on 26 November, a deal was close to being reached for Jack and Browne to fight on the Manny Pacquiao vs. Adrien Broner Showtime PPV undercard on 19 January 2019.
However, Jack found his way back into the fight, using his jab and body punches to render damage to his opponent and win rounds.
In the 12th and final round Jack returned the knockdown favor and dropped Pascal to the canvas with a overhand right and a flurry of right hooks.
[137] Jack took stay busy fight on Triller Pay-Per-View, on the undercard of Mike Tyson vs. Roy Jones Jr. exhibition in November 2020.
[139] In April 2021, the rematch against announced to take place on the undercard of Floyd Mayweather Jr. vs. Logan Paul on 6 June 2021 on Showtime PPV at the Hard Rock Stadium in Miami Gardens, Florida.
[155] Jack was then pitted against unbeaten Richard Rivera on the undercard of Oleksandr Usyk vs Anthony Joshua II on 20 August 2022.
[157] Jack then challenged Ilunga Makabu for the WBC cruiserweight title on the undercard of Jake Paul vs Tommy Fury on 26 February 2023.