In his NHL debut with Montreal on 19 November 1989, Cassels scored his first career goal on his first shift and first shot against the Calgary Flames.

He would explode in 1992–93, as he recorded 21 goals and 64 assists for 85 points, and formed a deadly partnership with sniper Geoff Sanderson establishing himself as one of the best young playmaking centres in the game.

While he would not come close to matching those totals again as Hartford struggled to non-playoff finishes, he continued to mature as an all-around player in the following years.

Cassels was a member of the final Whalers team in 1996–97, but did not make the move with the franchise to Carolina as he was dealt to the Calgary Flames that summer.

He would play two seasons for the Flames but struggled to produce in the defensive system of coach Brian Sutter, slumping to totals of just 44 and 37 points.

In the 2001–02, Cassels would play some of the best hockey of his career, recording 50 points in 53 games and helping the Canucks reach the playoffs - their second appearance since 1996 and his first since 1992.