ONE Championship In March 2018, Tonon made his professional mixed martial arts debut against Richard Corminal at ONE Championship: Iron Will, in
Bangkok, Thailand. Tonon won the fight via TKO in the second round. Tonon had his second professional fight against Rahul Raju at
ONE Championship: Reign of Kings on July 27, 2018. Tonon won the fight via rear naked choke in the third round. In November 2018, Tonon went on to fight Sung Jong Lee at
ONE Championship: Heart of the Lion. Tonon won via guillotine choke in the second round. On March 31, 2019, Tonon continued his winning streak and fought Anthony Engelen at
ONE Championship: A New Era. Tonon won via TKO in the first round. In May, Tonon faced Yoshiki Nakahara at
ONE Championship: Enter the Dragon He won the fight via a heel hook submission in the first round. Garry Tonon was scheduled to face Koyomi Matsushima at
ONE Championship: Inside the Matrix 4 on November 20, 2020. The bout was subsequently rescheduled for
ONE Championship: Big Bang on December 4, 2020 and it was confirmed that he would have to compete without his coach,
John Danaher, in his corner for the first time due to travel restrictions in place in Singapore. Tonon was unable to finish as Matsushima defended his submissions, but secured the unanimous decision victory to earn the first decision win of his mixed martial arts career. Tonon was scheduled to face reigning champion
Thanh Le for the
ONE Featherweight Championship at
ONE: X on December 5, 2021. In late October 2021, the promotion announced that the event was postponed due to the
COVID-19 pandemic. His title fight with Le was rebooked for
ONE: Lights Out on March 11, 2022. While attempting a heel hook, Tonon was caught by a hammer fist and lost by first-round knockout, handing him his first career MMA loss. Tonon faced Johnny Nuñez on January 14, 2023, at
ONE Fight Night 6. He won the fight via a kimura submission in the first round. Tonon faced Shamil Gasanov on July 15, 2023, at
ONE Fight Night 12. He submitted Gasanov with a kneebar in the second round and earned a Performance of the Night bonus. Tonon faced
Martin Nguyen on January 28, 2024, at
ONE 165. He won the match by rear-naked submission in the first round and earned a $50,000 'Performance of the Night' bonus. Tonon next faced Shamil Gasanov in a rematch at ONE Fight Night 34 on August 1, 2025. He lost the fight by decision. On April 24, 2026, it was announced that Tonon was no longer under contract with ONE Championship. ==Championships and accomplishments==