Cassidy owns a 117-52-22 record in 191 career games with Boston, good for the fourth-best winning percentage (.670) in team history. He required the second-fewest games among Bruins head coaches to record his first 100 wins with the franchise (166 games), trailing only Tom Johnson (138 games). Since taking the helm, Cassidy has led the Bruins to 256 points, second-most in the NHL over that span to Tampa Bay (283 points).

In just his second full season behind the bench in 2018-19, Cassidy led the Bruins to a berth in the Stanley Cup Final for the first time since 2013, following a regular season in which the team amassed a 49-24-9 record (107 points) and finished in second place in the Eastern Conference. It was the second-straight season the team hit the 100-point mark. The team ranked second in the NHL in wins (49), points (107), goal differential (plus-44) and power-play percentage (25.9). Cassidy also led the team on a 19-game point streak from January 29 - March 9, which was the second-longest streak in team history.

Cassidy led the Bruins to a 50-20-12 record in his first full season behind the bench in 2017-18, good for second place in the Eastern Conference and the fourth-best record in the National Hockey League. Under his guidance, the Bruins finished the year ranked in the Top 10 in the NHL in several key categories: first in wins when opponent scores first (21), second in shots allowed per game (29.3), second in shot attempts percentage (53.71), third in penalty kill percentage (83.7), tied for third in first-period goals (79), tied for third in third-period goals (95), fourth in power play percentage (23.5), fourth in goals against per game (2.57), sixth in goals per game (3.26), seventh in takeaways (682), eighth in hits (1,914), tied for eighth in shots per game (33.0) and tied for 10th in faceoff win percentage (50.7). Cassidy was also instrumental in the progression of several rookies, as first-year players accounted for an NHL-best 58 goals (21.7%) that season, eight more than the second-highest team total (Colorado, 50).

Cassidy served as interim head coach for the Bruins' final 33 regular and postseason games in 2017-18, 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. Once 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 54-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.