Kanehara had his MMA debut in a fight on May 29, 1998 against Lee Hasdell, winning by decision. He would follow with victories over Sander MacKilljan, Hans Nijman and Hasdell again, but his winning streak broke on February 1999 against Carlson Gracie apprentice Ricardo Morais. The 60 pound heavier Morais controlled Kanehara positionally with the aid of his size, leaving the Japanese relegated to try Kimura locks from underneath and trying to escape unsuccessfully. On May 22, Kanehara faced Valentijn Overeem from RINGS Holland. Again facing a heavier opponent, Kanehara was hit with knees and kicks to the head and downed in the initial minute, but he managed to execute a takedown on Overeem, who fell badly and dislocated his knee. Judges and referee deliberated about letting the match continue after the knee was relocated, and they eventually conceded. Returning to action, Overeem landed a body kick which Kanehara grabbed for another takedown, but Hiromitsu fell into a guillotine and was forced to spend a rope escape. The sequence repeated itself before Overeem knocked Kanehara out with a high kick. Kanehara had his debut in worldwide MMA at the 1999 edition of the King of Kings tournament, where he faced tough opposition. Kanehara eliminated Jeremy Horn in a prolonged grappling affair in the first round, winning the unanimous decisionthanks to his superior wrestling and ground work, and then went to face eventual winner Dan Henderson. The match was harsh and brutal, with Kanehara getting his nose broken and finally being dominated with repeated takedowns for a majority decision loss. After the tournament, Kanehara avenged his defeat to Valentijn Overeem, knocking out the Dutch fighter via punch from the clinch in a much shorter match. He was next sent as a RINGS representative to the event Colosseum 2000, where he found himself in front of Brazilian jiu-jitsu legend Mario Sperry; even at loss, Kanehara performed impressively, countering and escaping all the bad positions he was caught with by Sperry. Before the next edition of the tournament, Kanehara still fought the former finalist Renato "Babalu" Sobral, losing by decision. Kanehara returned to KoK format facing popular luta livre practitioner and ADCC Submission Wrestling World Championship champion Alexandre "Cacareco" Ferreira. Although believed to be technically outmatched, Kanehara fought a long, strategic match and ended submitting the decorated grappler by kimura. He then won his next match, quickly knocking out Tommy Sauer with punches, and reached the tournament's final event. There, Kanehara fought and defeated another tough opponent in the form of Dave Menne, dominating the stand-up and knocking him out for a TKO win in round 3, although there was some controversy when Menne's corner protested the stoppage had been premature. Finally, Kanehara faced the eventual winner, Brazilian jiu-jitsu expert Antonio Rodrigo Nogueira, losing a very back and forth grappling contest when he was caught in a choke and forced to submit. After his KoK tenures, Kanehara went to fight another Brazilian exponent and ADCC winner, Ricardo Arona, who he had already wrestled in ADCC in a losing effort. Though Kanehara was able to survive several striking combinations, holding his own on the mat, he was reversed in one of his own kneebar attempts and submitted. Right after he fought future Ultimate Fighting Championship welterweight champion Matt Hughes, losing by unanimous decision.
At the twilight of his career, Kanehara debuted in PRIDE Fighting Championships as a RINGS veteran, and was pitted against a series of world-level opponents. The first of them would be Wanderlei Silva in a challenge fight in PRIDE 23 for the PRIDE Middleweight Championship, but Kanehara was knocked down with strikes and hit with soccer kicks and stomps until his corner threw the towel. Hiromitsu returned to PRIDE as part of the Bushido series, taking on another feared striker in Mirko Cro Cop. The match went to the judges this time, as Kanehara absorbed a surprising amount of punishment, including Mirko's dreaded left high kick. He got similar results against Alistair Overeem in PRIDE 28; he survived a barrage of punches, flying knees and even a German suplex-like takedown, until he was finished by doctor stoppage. His final appearance in PRIDE was in its 29th event, being defeated by Maurício Rua via TKO in the first round.
