A professional competitor from 2001 to 2013, Chonan competed for the UFC, PRIDE, DEEP, DREAM, Pancrase, and World Victory Road.
Chonan officially began his professional mixed martial arts career in 2001, losing his first fight in Pancrase – Neo Blood Tournament.
Chonan fought in PRIDE Bushido 3 as a representative of Team Japan, losing a decision to Ricardo Almeida.
Chonan's next win was against future UFC Middleweight Champion Anderson Silva at PRIDE: Shockwave 2004, where he executed a spectacular flying scissor-lock takedown, followed quickly by a heel-hook submission.
He also lost his next fight against future teammate Dan Henderson via KO at the opening seconds of the bout at PRIDE Bushido 9.
In the 2006 PRIDE Welterweight Grand Prix, Chonan suffered a broken orbital bone at the end of his match against Joey Villasenor caused by a stomp but was able to continue in the tournament.
Chonan is currently a part of the Team Quest, along with former PRIDE Fighting Championships Middleweight and Welterweight Champion Dan Henderson, Matt Lindland, Jason "Mayhem" Miller, and Rameau Thierry Sokoudjou.