[7] After spending a year in refugee camps, at the age of four,[8][9] Mousasi and his family relocated to Leiden, Netherlands where he finished grade school before developing an interest in martial arts.

After losing to Gono, Mousasi fought Héctor Lombard in the Grand Prix Alternate bout and took a unanimous decision victory over the Cuban-born fighter.

[13] There he fought and submitted K-1 veteran Melvin Manhoef in the semifinals before meeting up with submission specialist Ronaldo Souza in the final.

"During the time I spent under their wing, M-1 Global, as a promoter and management company, allowed me to achieve many great things.

[20] Mousasi was then expected to face Rameau Thierry Sokoudjou at Dream 11, but he had to step down due to injury.

[22] It was originally planned for him to face Vitor Belfort at that event, but there was a disagreement between both fighters over which weight class the fight would be fought at.

[26] Mousasi was originally slated to defend his Strikeforce Light Heavyweight Championship, but eventually competed in a non-title bout.

[28] On April 17, 2010, Mousasi lost the Strikeforce Light Heavyweight Championship belt in his first defense to Muhammed Lawal by way of unanimous decision.

After the fight in an interview with Ariel Helwani of MMA Fighting.com, Strikeforce CEO Scott Coker stated “I’ve got a lot of respect for Keith Jardine, but I think Gegard won the fight.”[37] In the same interview, Coker also stated that a rematch between Mousasi and Jardine was definitely a possibility for the future.

Mousasi was expected to make his promotional debut against Alexander Gustafsson on April 6, 2013 at UFC on Fuel TV 9.

[47] However, on March 30, Gustafsson suffered a cut in training and, on April 2, was deemed unclear to participate by the Swedish MMA Federation.

[49] Mousasi won by unanimous decision, avoiding his opponent's takedown attempts and dominating the fight on the feet.

[62] After dominating the opening round, Mousasi was finished early in the second after absorbing a jumping spinning back kick, flying knee and follow-up punches.

[75] In the second round, Mousasi hit Weidman with two knees to the head, the latter of which had referee Dan Miragliotta interrupt the fight because he mistakenly thought it was illegal.

However, upon consultation with another referee "Big" John McCarthy, both knees appeared to be legal under the new unified rules introduced since 2017.

[80] On 10 July 2017, Mousasi announced that he had signed with Bellator MMA for a six-fights deal and should compete for the middleweight title later in 2017.

[82] The rest of the first round mainly consisted of Mousasi scoring multiple takedowns, landing many strikes, and attempting several neck cranks and rear naked chokes.

In the third and final round, Schlemenko landed the better of strikes standing up and Mousasi's only takedown attempt was unsuccessful.

[85] After scoring a takedown early, Mousasi used ground and pound to win the fight via TKO in the first round.

[88] Mousasi stuffed MacDonald's takedown attempt and delivered ground and pound, eventually winning the fight via technical knockout in round two.

[89] Mousasi was expected to make his second title defense against Rafael Lovato Jr. in the co-headliner of Bellator 214 on 26 January 2019.

[93] After losing the title, Mousasi rematched prior UFC opponent Lyoto Machida at Bellator 228 on 28 September 2019.

[95] In February 2020, it was announced that Rafael Lovato Jr. was forced to relinquish the Bellator Middleweight World Championship due to a cerebral cavernoma.

[100] Mousasi made the first title defense of his second reign against John Salter on August 13, 2021 at Bellator 264.

[102] Mousasi made his second middleweight title defence against Austin Vanderford at Bellator 275 on February 25, 2022 at the 3Arena in Dublin, Ireland.

[104] Mousasi attempted his third middleweight title defence against Johnny Eblen at Bellator 282 on June 24, 2022 [105] He lost the bout via 50-45 unanimous decision on all judges' scorecards.

[113] On January 24, 2025, Mousasi was Team London's first draft pick in the inaugural edition of Global Fight League.

[114] In somewhat of a surprise to the MMA world, Mousasi agreed to take on Japanese K-1 fighter Musashi in a K-1 rules fight at Dynamite!!

Mousasi was the rank outsider to win the match against the more experienced Musashi, but came out fast and scored a first-round KO.

[117] His older brother Gewik was a mixed martial artist before him, and Gegard decided to follow in his footsteps.