Colaiacovo was drafted in the first round,17th overall, by the Toronto Maple Leafs in the 2001 NHL Entry Draft. Colaiacovo has been bothered by injuries that have hindered him from becoming an NHL regular, but many still consider him to be a good prospect. He scored his first career NHL goal on November 8, 2005 against the Washington Capitals.
On January 23, 2006, Colaiacovo missed a hipcheck on Vaclav Varada in a game against the Ottawa Senators, smashing head-first into the boards. He was taken off the ice in a stretcher. The injury was a major concussion, and forced Colaiacovo to miss the end of the 2005-06 NHL season.
Colaiacovo recovered fully from his concussion, but later injured his hand in his first game back into the Toronto Marlies lineup. On December 4, 2006, the Maple Leafs recalled Colaiacovo from the Marlies, and on December 16, he scored his first goal of the season against the New York Rangers. Colaiacovo underwent arthroscopic knee surgery on April 29, 2007.