Gegard Mousasi began competing in mixed martial arts in 2003 at the age of 18, running his professional record to 12-1-1 before debuting for the Japan-based Pride Fighting Championships in 2006. Mousasi competed in the Pride 2006 Welterweight (183 lbs.) Grand Prix, but was defeated in the tournament semifinals by Japanese standout Akihiro Gono by Armbar. Since his loss to Gono, Mousasi has established himself as one of the premier fighters in the world, with notable wins over Hector Lombard, Evangelista Santos, Denis Kang (Triangle Choke), Melvin Manhoef (Triangle Choke), Ronaldo Souza, Renato Sobral, and Remeau Thierry Sokoudjou, winning the Dream Middleweight Grand Prix along with the Strikeforce Light Heavyweight Championship in the process.

One of the most effective finishers in active competition, Mousasi has been to decision only a handful of times in his professional career, preferring instead to finish his fights either by knockout, TKO, or submission. After spending the first part of his career competing in the middleweight division, Mousasi has since alternated between the light heavyweight and heavyweight divisions, citing his growing frame and the hunger for greater challenges as reasons for his move.