Out of college, Cassidy was signed by the Toronto Blue Jays. During his first professional season in Medicine Hat, he made the Pioneer League All-Star Team. While with Single-A Dunedin in the Florida State League in 2000, he threw a no-hitter against Fort Myers.
Over the course of his major-league career (2002 and 2005-06), Cassidy's record was 8-9, with 4.88 ERA. He pitched primarily for the Toronto Blue Jays and the San Diego Padres but also had stints with the Boston Red Sox and Milwaukee Brewers. While in Toronto's organization, he pitched for the Syracuse SkyChiefs in 2001,'02 and '03.
After retiring from pro ball during spring training with the Brewers in 2008, Cassidy accepted a head coaching position at Sullivan County Community College, in the Catskills. In 2009, he became pitching coach and recruiting coordinator at Le Moyne.
Le Moyne Athletic Director Matt Bassett called Cassidy "a perfect fit" as the college's new baseball coach.
Cassidy lives in the village of Liverpool with his wife, Melissa. He's the son of Mike and Barbara Cassidy of Clay.