Colosimo completed the Australian Institute of Sport football program in 1997 and signed with new National Soccer League club Carlton for their inaugural year. Interest had been shown from Bayern Munich and Panathinaikos to sign Colosimo, but a knee injury sustained in a horror tackle by Andrew Cole while on Socceroos duty kept him out of the game for most of the 1999/00 season, but made his return on 14 March 2000 to score against South Melbourne, who would eventually acquire the midfielder after the demise of the Carlton club.