As a youth, he played in the 1982 Quebec International Pee-Wee Hockey Tournament with a minor ice hockey team from Chinguacousy in Brampton, Ontario. Andrew Cassels was selected 17th overall by the Montreal Canadiens in the 1987 NHL Entry Draft. He played three stellar seasons with the Ottawa 67's of the OHL, his best season coming 1987–88, when he led the OHL in assists and points in both the regular season and playoffs en route to winning the league MVP award. 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. However, Cassels would find his playing time limited on a deep Montreal team, and only played 60 games for the Canadiens before being traded to the Hartford Whalers in 1991 for a second round draft pick that the Canadiens used to pick Valeri Bure. It was in Hartford that Cassels had his greatest success. He recorded 41 points in 1991–92 campaign and had a strong playoffs, finally showing the offensive potential that had seen him taken in the first round of the draft. 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 following years and he and Sanderson remained a consistent and effective offensive partnership. Cassels led the Whalers in assists for 5 consecutive seasons from 1992–1997, and led the team in overall scoring in the lockout-shortened 1994–95 season. 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 1999 Cassels became an unrestricted free agent and signed a three-year contract with the Vancouver Canucks. In Vancouver, he would be rejuvenated offensively, bouncing back to record 45 assists and 62 points in 1999–2000. In his first two seasons in Vancouver, he would lead the team in assists and finish second in scoring. However, team success continued to elude him, as his string of playoff misses was extended to nine. In 2001–02, Cassels would play some of the best hockey of his career, recording 50 points in 53 games in helping the Canucks reach the playoffs - their second appearance since 1996 and his first since 1992. With the expiration of his contract in 2002 and the development of younger centremen Brendan Morrison and Henrik Sedin, Cassels left Vancouver in 2002 to sign a free agent contract with the Columbus Blue Jackets, where he was re-united with his friend and former linemate Geoff Sanderson. None of their chemistry had disappeared in their five years apart, as both players had their biggest years since their time in Hartford. Cassels' 48 assists and 68 points were his highest totals since his career year in 1993. However, injuries would take their toll in the 2003–04 NHL season as he slumped to just six goals and 26 points, his lowest totals since his rookie year in Montreal. Following the 2004–05 NHL lockout, Cassels signed with the Washington Capitals on 12 August 2005. However, the year-long layoff, along with age and injuries, had greatly reduced his effectiveness. After playing just 31 games and scoring only 12 points, Cassels was released by the Capitals on 28 January 2006. In 16 NHL seasons, Cassels appeared in 1015 games, recording 204 goals and 528 assists for 732 points along with 410 penalty minutes. He led his team in assists in seven of those seasons.
