In 1992 he played in the World Junior Ice Hockey Championship for the United States. Brennan's most successful collegiate season came during his junior year (1993–1994) when he scored 35 points in 41 games and earned a number of honors including Hockey East First All-Star team honors and NCAA East Second All-American team honors.
Brennan would turn pro in 1995 after graduating with a bachelor of arts from BU. His pro career was long and well travelled, though mostly played in the minor leagues and in Europe. He did for a number of NHL teams in his 14-year pro career, including the Colorado Avalanche, San Jose Sharks, New York Rangers, Los Angeles Kings, Nashville Predators, and Boston Bruins. In total he played 50 NHL games, scoring two goals and eight points.
Brennan retired to the Boston area and became a sales manager for a Xerox subsidiary company named Image Tech.