Kenna made his professional boxing debut in 1991 against Alvin 'Slick' Miller, where he was knocked down in the first round but rallied and stopped Miller on a third-round TKO. A nine-fight undefeated streak followed with eight wins and a draw. The fights in this streak were all against journeymen fighters until he fought Michael Sprott for the Southern Area Heavyweight Title. Colin was defeated for the first time in this fight with a first Round KO. Three successive wins followed before Kenna faced Audley Harrison's victim, Mark Krence, who at the time was highly rated and tipped as a future star. An injury to his elbow forced a corner retirement.