Cassels was coveted by the Montreal Canadiens while playing junior in Ottawa, particularly after 1987-88 when he had a league-leading 103 assists and 151 points. The Habs had drafted him the previous summer, 17th overall, and another year later he played for Canada at the 1989 World Juniors.
The OHL's MVP in 1988, Cassels started his first NHL games the following year with the Habs but spent the majority of the year in the minors. The next year, he was back for good, but the Habs gave up on him after he managed just six goals. He was traded to Hartford and blossomed into a decent scorer, three times hitting the 20-goal mark. After six seasons in Hartford, Cassels was sent to Calgary in 1997-98 and then moved to Vancouver in 1999-00, all the while contributing offensively.
A silver medalist with Canada's World Championship team in 1996,Cassels signed as a free agent with the Columbus Blue Jackets in 2002-03, after three seasons in Vancouver. Upon his arrival with Columbus, Cassels became one the team's leaders both on and off the ice and in 2003-04 surpassed the 700-point plateau for his career.
Following a lock out season in 2004-05, Cassels was acquired by the Washington Capitals in the summer of 2005. However, Cassels was released by the club midway through the season.
In 16 NHL seasons, Cassels saw action in 1015 games and recorded 204 goals and 528 assists for 732 points.