Gustafsson spent a major part of his MMA career in the Light Heavyweight division of the UFC, where he became a top contender and fought for the title three times.

[17][18] The most notable victories in his first 8 fights came against German prospect Florian Muller, who he defeated by TKO in the second round on 13 March 2008.

[19] Later that year, after another TKO victory against Matteo Minonzio, he replaced his former opponent Florian Muller on less than 24 hours notice against Krzysztof Kulak, who at the time was a top prospect in Poland, on 13 September 2008, at KSW Extra.

[20] In his last fight before he was signed to the UFC, Gustafsson fought European veteran Vladimir Shemarov on 30 May 2009 at Superior Challenge 3.

[30] In advance of this bout, Gustafsson traveled to San Diego, California, to train with Alliance MMA for a month.

[32][33] In post-fight interviews Gustafsson said that the main reason for his success against Diabate was the time that he spent at Alliance MMA in the training camp.

[43][44] After the win UFC President Dana White said that a first round finish of Matyushenko, who he referred to as a gatekeeper at the time, made it clear that Gustafsson was a force to be reckoned with and that he would get a step up in competition for his next fight.

[48] Just like after his previous win, it was again clear that Gustafsson was in need of another step up in competition, and UFC President Dana White said that his next fight would be against someone in the top of the rankings.

[53][54] Gustafsson was expected to face former Strikeforce Light Heavyweight Champion Gegard Mousasi on 6 April 2013, at UFC on Fuel TV 9[55] and it was said that a victory would ensure him of a shot at the title.

[81] At a post-fight press conference in Manchester, Dana White said that Gustafsson would be facing Antônio Rogério Nogueira on 8 March 2014, at UFC Fight Night 37.

[90] On 27 April 2014, UFC President Dana White confirmed that a rematch with the champion Jon Jones would be next for Gustafsson, and stated the possibility of the match taking place in a stadium in Sweden on pay-per-view.

[100] However, Gustafsson was forced to pull out of the bout due to a torn meniscus and was replaced by fellow top contender Daniel Cormier.

[106] Gustafsson instead faced Anthony Johnson at the event with the winner guaranteed a title shot against Jon Jones.

[107] Gustafsson lost the fight via TKO in the first round, marking the first time in his career that he had been stopped due to strikes.

[123] A pairing with Antônio Rogério Nogueira was scheduled for a third time and was expected to take place on 19 November 2016, at UFC Fight Night 100.

In his post-fight interview, Gustafsson announced his retirement from MMA by taking off his gloves, laying them on the floor of the octagon, and saying "The show is over, guys".

[145][146][147] On 7 November 2019, UFC President Dana White confirmed that Gustafsson was showing interest in returning.

[148] On 11 December 2019, Gustafsson competed in a grappling match in Sweden, against Swedish MMA prospect Anton Turkalj.

[150] In an interview on 3 April 2020, Gustafsson revealed that he was planning a training trip to the United States as soon as the travel restrictions due to the COVID-19 pandemic were lifted.

[151] On 5 June 2020, Gustafsson announced his return to active MMA competition in the UFC as part of the heavyweight division.

[154] Gustafsson was scheduled to face Paul Craig in a light heavyweight bout on 4 September 2021, at UFC Fight Night 191.

[176][177] At numerous occasions, Alliance fighters has also travelled to Sweden to be a part of Gustafsson's training camps.

[178][179] In addition to Gustafsson and his long time training partner Ilir Latifi,[180] the Allstar team has also added other big names from the heavier divisions.

[189][190][191] He grew up in Arboga, a small town about 1+1⁄2 hour drive west of Stockholm, together with his mother, step-father and four younger sisters.

[193] His biological father died a few weeks prior to Gustafsson's fight in London against Jimi Manuwa on 8 March 2014.

[194][195] His nickname, "The Mauler", was given to him early in his career by training partners, referring to the powerful striking, killer instinct and aggression to finish fights.

[198] He is a long time friend, and former occasional training and sparring partner, of two-division boxing world champion Badou Jack.

[203][204] He was released in 2006, turned his life around, got back into training, found MMA and moved to another city to leave the trouble behind him.

[208] Gustafsson played a role in the Swedish mainstream movie Johan Falk – Blood Diamonds which premiered in 2015.

[209][210] As of 2019, Gustafsson is a part owner at the Swedish mixed martial arts promotion AK Fighting Championship.