He holds wins over Tito Ortiz, Masakatsu Funaki, Yuki Kondo, Semmy Schilt, and Minoru Suzuki.
Mezger has trained with many great martial arts competitors and trainer/instructors; his main trainers have been Vince Tamura (judo), Willie Thompson (wrestling), Billy "Jack" Jackson (kickboxing), and Ken Shamrock (shootfighting).
Mezger has co-written a book, The Complete Idiot's Guide to Kickboxing; he had an uncredited speaking role in the first-season episode of Walker, Texas Ranger titled "Night of the Gladiator".
Born in Houston and raised in Dallas, Mezger wrestled in high school and also practiced taekwondo, in which he holds a 7th degree black belt.
[2] Mezger began his mixed martial arts career in the Ultimate Fighting Championship at UFC 4 in an alternate match against Jason Fairn.
Mezger next fought at UFC 5 in an alternate match against John Dowdy, defeating him by TKO in little over two minutes by mounted strikes.
The American fighter performed well, scoring high kicks and forcing Funaki to pull guard and work from the bottom, but he was eventually caught in a leglock exchange from his own trip and submitted with an Achilles lock.
In December 1995, Mezger was involved in a controversial match against the other Pancrase co-founder, lauded wrestler Minoru Suzuki.
Leininger was able to score a takedown and gain mount at one point, but he was otherwise soundly beaten as Mezger battled his way to a decision victory.
The championship round Mezger faced future UFC Light Heavyweight Champion Tito Ortiz.
Later that night, the president of Pride FC made a public apology to Mezger at the Tokyo Dome for the miscommunication.
[6] Mezger talked about his feelings on the matter in an interview: "I am not going to cry foul, it is the fight game and things like that happen, get used to it.
"[2] Mezger found a measure of redemption when he defeated Alexander Otsuka by TKO at Pride 12 - Cold Fury.
Mezger then met UFC Hall of Famer Chuck Liddell at Pride 14 - Clash of the Titans.
Mezger entered the ring with an American flag draped across his shoulders out of respect for the World Trade Center attacks in New York.
[8] Thus, Mezger returned to competition at Pride 22 after a year long lay-off, easily winning a decision over Norihisa Yamamoto.
Mezger had several good striking exchanges and showcased his submission and takedown defense, but again lost the fight by a controversial split decision.
On January 25, 2005, Guy Mezger retired from professional fighting after experiencing stroke-like symptoms prior to his scheduled bout with Tito Ortiz.
Mezger is a part owner in a movie/TV production company that is making "reality TV" shows and action and documentary movies.
He has also commentated for Chuck Norris' full contact, team-based martial arts competition, the World Combat League, and for Japanese MMA organization DREAM.
[10] In December 2011, Mezger was involved in an altercation in Dallas in front of a sporting goods store where a man was physically abusing a woman.