Graham started his career in Kyokushin Karate and won the Amateur Australian title in 1999, in the open weight division. Before turning pro Graham captured the WKBF World Amateur Heavyweight title from New Zealand's Shane Wijohn, finishing his amateur career undefeated (12-0). He turned pro in 2000 and attained instant success. He defeated Ben Hamilton in K-1 Oceania Revenge by TKO and then Clay Aumitagi.

On November 19, 2000 he became the first Australian to go ten rounds with the legendary Stan "The Man" Longinidis. Though losing first fight of his career, Graham learnt more from that fight than any other of his young career.

In 2006, Peter did also had a feud with Badr Hari in K-1 World Grand Prix 2006 in Auckland. Hari mocked Graham that he is a has-been, and Hari is the future. A brawl at the press conference instigated by Hari kissing Graham hyped up their match. The real match was won by Peter, via his trademark "Rolling Thunder" spinning kick, which broke Hari's jaw in 3 places. Hari was sidelined for almost a year.

On December 31, 2006 at K-1 PREMIUM 2006 Dynamite!! Peter Graham fought against the two time K-1 heavyweight champion Semmy Schilt in a 5 round match and lost by unanimous decision (3-0).

On August 5, 2007, Sunday, Graham and Hari were tied for a revenge fight in K-1 World GP 2007 in Hong Kong as one of the three super bouts of the event which was held in Hong Kong for the very first time. In this occasion, Hari was more alert in case of any KO defeat of a Rolling Thunder by Graham. Both fighters could not show their top performances as Hari won the fight by points. Graham tried Rolling Thunder Kick twice in the last moments of the fight but he could not connect Hari.