He first joined the Bruins organization in 2008 as an assistant coach in Providence and was named the tenth head coach in that team's history on June 25, 2011. He compiled a 207-128-45 record at the helm of the AHL Bruins, including the best record in the AHL in 2012-13 at 50-21-5 and second-place division finishes in each of the last two seasons.
The native of Ottawa, Ontario has a lengthy resume from both his playing and coaching careers. Selected 18th overall by the Chicago Blackhawks in the first round of the 1983 NHL Entry Draft, he played with the Blackhawks and their affiliates from 1985 to 1990, including 36 Chicago games with 4-13=17 NHL totals. He then played three seasons in Europe, suiting up for Alleghe HC in Italy and Kaufbeuren in Germany. He retired with the IHL's Indianapolis Ice in 1996 to accept a head-coaching job with the Jacksonville Lizard Kings of the East Coast Hockey League (ECHL).
Cassidy was promoted to head coach of the team in Indianapolis during the 1998-99 season and went on to coach the ECHL's Trenton Titans and the IHL's Grand Rapids Griffins before being hired as head coach of the Washington Capitals in 2002-03. The Capitals had an overall record of 47-45-9-6 under his tenure.
Cassidy signed with Chicago in June 2004 as an assistant coach, working with the AHL's Norfolk Admirals during the 2004-05 NHL lockout season and then the Blackhawks in 2005-06. He again became a head coach in 2006-07, guiding the OHL's Kingston Frontenacs to a 31-30-7 mark and a trip to the playoffs. His time in Kingston concluded with a two-year record of 33-39-8, bringing his overall head coaching record to 295-253-36-37 in 621 games.
Cassidy and his wife Julie have two children, daughter Shannon and son Cole.