Cassidy served as Interim Head Coach for the Bruins' final 33 regular and postseason games, compiling a 18-8-1 regular season record and propelling the team to a berth in the Stanley Cup Playoffs for the first time since the 2013-14 season. Since Cassidy assumed head coaching responsibilities on February 9, the Bruins ranked first in the NHL in goals per game (3.37), first in the NHL in fewest shots allowed (741), tied for second in the NHL in wins (18), tied for second in the NHL in power play percentage (27.8%), tied for third in the NHL in goals allowed per game (2.30), tied for fifth in the NHL in faceoff percentage (53.6%) and tied for sixth in the NHL in takeaways (229).

Before joining Boston as an assistant prior to the start of the 2016-17 season, Cassidy spent five seasons (2011-16) as head coach of the Providence Bruins, having spent the three previous seasons (2008-11) with the club as an assistant. The 51-year-old native of Ottawa, Ontario compiled a 207-128-45 overall record in 380 games at the helm, including winning seasons in all five years and postseason berths in each of his final four seasons in Providence. In 2015-16, Cassidy helped lead the P-Bruins to a 41-22-13 record.

Cassidy's previous NHL experience includes coaching the Washington Capitals from 2002-04, as he led the Caps to a 39-29-8-6 record and a postseason berth in his first season with the team. Following his exit from Washington, he served as an assistant coach with the Chicago Blackhawks for the 2005-06 season.

Cassidy also enjoyed a successful playing career, seeing action in the OHL, AHL, IHL and NHL, as well as overseas in Italy and Germany. The left-handed defenseman was originally drafted in the first round (18th overall) of the 1983 NHL Entry Draft by the Chicago Blackhawks. Over the course of five seasons (1985-90) at the NHL level, Cassidy recorded four goals and 13 assists for 17 points with 10 penalty minutes in 36 games played.