In his first season in North America, Ullmark attended the Sabres' training camp. After recovery from an early injury, he was assigned to begin the 2015–16 season with the Rochester Americans of the American Hockey League (AHL). Ullmark played in three games before being recalled to the Sabres on 23 October 2015. He made his NHL debut the following day, making 24 saves in a 4–3 defeat to the New Jersey Devils.[3] His first victory came in his second start, a 3–1 win over the Philadelphia Flyers.[4]
Ullmark would spend majority of the subsequent season with the Americans, amassing a record of 26–27–2. He appeared in one game for the Sabres, falling 4–2 to the Tampa Bay Lightning. On 13 June 2017, the Sabres re-signed Ullmark to a two-year, $1.5 million contract with an annual average of $750,000.[5]
Ullmark joined the Sabres full-time during the 2018–19 season. In his season debut on 13 October, he recorded his first career NHL shutout, stopping all 36 shots faced in a 3–0 win over the Arizona Coyotes.[6] On 22 December, Ullmark stopped all 40 shots faced to shut out the Anaheim Ducks 3–0.[7] Overall, he recorded a 15–14–5 record in 37 games while backing up Carter Hutton.
On 3 August 2019, the Sabres re-signed Ullmark to a one-year, $1.325 million contract.[8] On 25 October 2020, Ullmark signed a one-year, $2.6 million contract with the Sabres.
Boston Bruins
On 28 July 2021, Ullmark left the Sabres as a free agent and signed a four-year, $20 million contract to be the starting goaltender with the Boston Bruins.[9]
Ullmark played his first regular season game for the Bruins on 22 October 2021, a 4–1 win against the Sabres.[10] Although rookie Jeremy Swayman started the first couple games of the season, Ullmark started most of the games before Tuukka Rask's return from injury. Ullmark was named as the starting goaltender for the Bruins for their first round series against the Carolina Hurricanes in the 2022 Stanley Cup playoffs. Ullmark would struggle in the first two games, as he let up four goals in both games, both of which were losses. As a result, the team opted to start Swayman in Game 3, who would start the rest of the series. The Bruins would be eliminated by Carolina in Game 7.
The 2022–23 season was a historic one for the Bruins. Ullmark and Swayman's post-victory "goalie hug" embrace became a widely-covered team ritual.[11][12] Ullmark was named one of three Boston representatives to the 2023 NHL All–Star Weekend, joining head coach Jim Montgomery and teammate David Pastrňák. Ullmark notched his 100th career win on 19 January, saving 25 of 26 shots against the New York Islanders.[13] On 25 February, he became the thirteenth NHL goaltender to score a goal, and the first since Pekka Rinne in 2020, as he shot the puck directly into the Vancouver Canucks' empty net with less than one minute remaining in the Bruins' 3–1 win.[14] The Bruins finished the regular season with new records in both wins (65) and points (135), winning the Presidents' Trophy. Ullmark and Swayman together received the William M. Jennings Trophy as the goaltending tandem to allow the fewest goals against.[15][16] Ullmark's personal record was 40–6–1 and he led the NHL in GAA (1.89) and save percentage (.938). He was named a finalist for the Vezina Trophy as the league's best goaltender.[17] The Bruins entered the 2023 Stanley Cup playoffs as the favourites for the championship. However, they were ousted in the first round by the Florida Panthers, squandering a 3–1 series lead in the process. Ullmark was ultimately pulled after Game 6, after he had allowed seven goals in the game.[18][19] Ullmark defended coach Montgomery, saying "something that everybody does when things aren't going the way they want to is they're trying to find a scapegoat."[20]
Although Ullmark entered the 2023–24 season as Boston's starting goaltender, Swayman's play saw him get an uptick in starts. As was established in the previous two seasons, however, the two still split relatively even starts, with Ullmark starting 39 games to Swayman's 43. Ullmark finished off the season on a strong note, recording a 1.90 GAA and .935 save percentage in his final nine games after the deadline. During the 2024 Stanley Cup playoffs, the Bruins were eliminated in six games by the Panthers in the second round.[21]
Ullmark is a fan of esports, and of Dota 2 in particular. On 18 March 2019, he appeared on a Dota 2 podcast where he discussed the differences between playing Dota and ice hockey, as well as comparisons between the professional scenes of competitive gaming and traditional sports.[23]