Mezger started his mixed martial arts career in the Ultimate Fighting Championship at UFC 4 in an alternate match against Jason Fairn. Before the fight, Mezger had asked Fairn to make a 'gentlemen's agreement' not to pull hair due to the fact both men had long hair. Mezger won the bout by TKO after landing a multitude of strikes from full mount. Mezger then fought at UFC 5 in an alternate match against John Dowdy, defeating him by TKO in little over two minutes by mounted strikes. Mezger began to train with Ken Shamrock and became a member of Ken's submission fighting team, the Lion's Den, and joined the Pancrase organization in Japan.