Like many a teenager, Cassidy put up big numbers during his junior days and attracted plenty of attention from NHL scouts. Chicago selected him 18th overall at the 1983 Entry Draft and the next season he helped the Ottawa 67's to the Memorial Cup and also got into his first game with Chicago.
A serious knee injury, though, curtailed his development and through to 1990 Cassidy never played more than 21 games in any season with the Hawks. Instead, he was in the minors most of the time, his scoring ability diminished and his confidence not up to the NHL level.
Feeling his career was stagnating, Cassidy took off for Europe and played in Italy and Germany for the next four years until re-signing with Chicago and returning to North America. But the return was minor league, and he played the next three seasons in the IHL before becoming coach of the Jacksonville Lizard Kings in the ECHL, a post he has held up until 1998.
After two seasons as head coach in Jacksonville, Cassidy returned to the Indianapolis Ice his last team as a player and became the head coach. One year later Cassidy was on the move again, this time with the ECHL's Trenton Titans before signing on with the IHL Grand Rapids Griffins in 2000-01. Two years into his stint as head coach of the Griffins, Cassidy was hired on as head coach of the Washington Capitals in the summer of 2002 before ultimately being dismissed midway through the 2003-04 season.