During his time at BC High, Leahy set a school record for points in career (with 140), was named to the Boston Globe and Boston Herald Dream Teams as a senior, and was honored with the titles of team captain and MVP.

During his first year of professional hockey, Leahy played for four pro teams, including the Trenton Titans of the ECHL, the Hershey Bears, the Portland Pirates and the Bridgeport Sound Tigers of the American Hockey League.

On July 17, 2006, Leahy left the Bruins organization and signed a one-year contract as a free agent with the Nashville Predators.

Making just a single appearance with the Predators during the 2006–07 season, Leahy was assigned to AHL affiliate, the Milwaukee Admirals of the majority of the year.

Unable to establish an NHL spot after six professional seasons, Leahy signed with European team EHC Black Wings Linz of the Austrian Hockey League.