His style incorporates a low and wide stance, a unique shin-lock RVH positioning, and effective rebound control.

Rask played started as the primary goaltender for the Ilves senior team in the Finnish top-flight SM-liiga in 2004.

However, before playing a regular season game for Toronto, he was traded to the Boston Bruins in exchange for former Calder Memorial Trophy-winning goaltender Andrew Raycroft.

Toronto management instead chose to keep goaltender Justin Pogge, who had just won a gold medal with Team Canada at the World Junior Championships.

Rask had the best save percentage (.952) among the goalies in pre-season play, followed by teammates Manny Fernandez (.875), Tim Thomas (.869) and Kevin Regan (.857).

With nagging back spasms keeping Fernandez from play shortly after the All-Star Game break, Rask was once again called up to serve as a second goaltender, and on 31 January 2009, he played his first (and only) game with the Bruins in the 2008–09 season, and earned his first ever NHL shutout, a 1–0 home effort against the New York Rangers, with Marc Savard scoring the only Bruins goal.

[20] In the 2010–11 season, Tim Thomas returned to top form, effectively relegating Rask once again to the backup role.

Prior to the declaration of the 2012–13 lockout, Rask was named as the starting goaltender for the Bruins, replacing Tim Thomas, who would eventually be traded to the New York Islanders on 7 February 2013.

[24] During the lockout, which ended on 6 January 2013, Rask played for HC Plzeň, which won the Czech Extraliga that year.

In the Stanley Cup Finals, the Bruins were defeated in six games by the Presidents’ Trophy-winning Chicago Blackhawks, as Rask registered a .932 save percentage.

He posted a 36–15–6 record, highlighted by a career-best ten-game points-won streak from 20 to 30 March, going 9–0–1, as the Bruins captured their first Presidents' Trophy since 1990 and led the Eastern Conference in team defence (2.08 goals allowed per game).

Following a six-game quarterfinal series with the Bruins losing to the Ottawa Senators four games to two, Rask successfully underwent groin surgery on 9 May 2017.

[34] On 3 February against the Washington Capitals, Rask recorded a shutout to become the career leader for wins by a goaltender in Bruins history, again surpassing Tiny Thompson.

[42] Following the game 6 exit of the Bruins from the playoffs in the second round against the New York Islanders, Rask revealed he had been dealing with an early-season injury to an acetabular labrum in one of his hips, necessitating surgery during the summer and a likely return to play for the team.

[45] However, the games he was slated to start in were postponed due to a COVID-19 outbreak among the Lehigh Valley Phantoms, who were then unable to travel to Providence for the weekend's contests.

Despite missing those rehab starts, Rask signed a one-year contract with the Boston Bruins on 11 January, worth $1 million.

On 2 March 2016, it was revealed that Rask was to be the starting goaltender for Finland in the 2016 World Cup of Hockey, ahead of Pekka Rinne.

[51] In February 2015, a recently discovered species of wasp in Kenya was named Thaumatodryinus tuukkaraski as a direct reference to Rask.

"[52][53] Bold indicates led league In addition to the above, a newly discovered wasp species, Thaumatodryinus tuukkaraski, was named in Rask's honor in 2015.