Johnson made his MMA debut in 2002, going 4–4 by years end. Johnson then made his MFC debut losing to Chad Hanseh. Starting on October 2005, Johnson went 11–1–1 over five years with his only loss coming in a title fight for the KOTC Canadian Lightweight Championship against future UFC top contender and training partner Rory MacDonald. He also captured the XMMA Lightweight title. Johnson would make his return to MFC at MFC 23 taking on Ryan Machan. He lost via submission in the first round. Johnson was set to take on Orestes Betran at MFC 27. Betran then pulled out of the match and was replaced with a veteran of 29 fights at the time, Ryan Healy. He won via unanimous decision.
In late 2013 it was announced that Johnson will be one of the Canadian fighters picked to be on. In episode 1, Johnson is picked to fight first against Australia's own Brendan O’Reilly. Johnson states that he is injured, but that he always fights injured. Despite the injuries, Johnson implies he is still more dangerous than anybody else in the house. Johnson gives Canada their first win with a rear-naked choked in the first round. In episode 10, Johnson was set to face his teammate Chad Laprise in the semi finals. Johnson lost via KO in the second round. Johnson's jaw was broken from the punch. After the show Johnson was noted as the reason for the removal of fellow TUF Nations contestant Tyler Manawaroa from the UFC after claiming Tyler was a racist due to an image posted on his personal Instagram. Tyler was of native New Zealand Maori descent and later explained it was the actions of a schoolboy making fun of himself and his own heritage, as in Australia many Aboriginals, Maoris and Polynesians consider themselves 'dark-skinned' or 'non-white'. Kajan later apologised though nevertheless Tyler was removed from the UFC roster and remains inactive yet undefeated.
Ultimate Fighting Championship
Johnson made his promotional debut at UFC 174, taking on Tae Hyun Bang. He lost via knockout in the third round. They both earned Fight of the Night honors. Johnson was then set to face Zhang Lipeng on May 16, 2015 at UFC Fight Night 66. He won via unanimous decision. Johnson faced Naoyuki Kotani on September 27, 2015 at UFC Fight Night 75. He won the fight by unanimous decision. Returning from a near two year hiatus, Johnson faced Adriano Martins at UFC 215 on September 9, 2017. He won the fight via knockout in the third round. Johnson was scheduled to face Rustam Khabilov on March 17, 2018 at UFC Fight Night 127. However, Khabilov pulled out of the fight citing injury and was replaced by Stevie Ray. He won the fight via split decision. Johnson faced Islam Makhachev on July 28, 2018 at UFC on Fox 30. He lost the fight via armbar submission in the first round. The fight with Rustam Khabilov was rescheduled and eventually took place on September 15, 2018 at UFC Fight Night 136. Johnson lost the back-and-forth fight by split decision. On September 29, 2018, Johnson revealed that the fight with Khabilov was the last of his then-current contract and UFC had not made any gesture for re-signing Johnson.
