Florida Panthers

In his first professional season, Montembeault attended the Panthers 2017 training camp, before he was re-assigned to begin the 2017–18 season in the American Hockey League with affiliate, the Springfield Thunderbirds. He made his professional debut, making 25 saves for the Thunderbirds, in a 3–2 defeat to the Providence Bruins on October 13, 2017.^[3] He collected his first win in his 7th outing, backstopping the Thunderbirds in a 5–3 decision over the Hartford Wolf Pack on November 3, 2017.^[4] With the Thunderbirds finishing well out of the playoffs, Montembeault finished with 13 wins through 41 games.^[5]

Montembeault was assigned to continue in the AHL with Springfield to begin the 2018–19 season. After 33 games with the Thunderbirds, having already matched his previous win totals, Montembeault received his first recall by the Panthers due to starting goaltender Roberto Luongo briefly leaving the team on bereavement leave, on February 18, 2019.^[6] In his second recall to the Panthers following an injury to backup James Reimer, Montembeault made his NHL debut in his first career start with the Panthers in a 4–3 overtime defeat to the Carolina Hurricanes at the BB&T Center in Sunrise, Florida on March 2, 2019.^[7]

Montreal Canadiens

On October 1, 2021, Montembeault was placed on waivers by the Panthers. The following day, he was claimed by the Montreal Canadiens.^[8] Montembeault made his debut for the team in a pre-season game against the Toronto Maple Leafs, where the Canadiens lost 6–2, but Montembeault was widely credited for a strong performance despite weak defending by the team.^[9] Following starting goaltender Carey Price's taking a leave of absence for mental health reasons, Montembeault temporarily became the primary backup goaltender to Jake Allen.^[10] On January 12, 2022, Allen was injured in a game against the Boston Bruins and subsequently announced to be out for at least two months, with Montembeault becoming the Canadiens' starting goaltender in the meantime.^[11] The Canadiens named him their player of the month for January, having become the first goaltender in team history to post two consecutives games with 48 saves or more.^[12] He earned his first NHL shutout win against the Buffalo Sabres on February 23, 2022.^[13] Upon Allen's return he played only sparingly, but after Allen's re-injury in early April he was again the team's starting goaltender, pending the return of Price.^[14] Following the conclusion of the season, Montembeault underwent wrist surgery.^[15]

On July 18, 2022, Montembeault was re-signed to a two-year, $2 million contract extension with the Canadiens.^[16] Beginning the 2022–23 season as Allen's backup, he was soon noted for markedly improved performance, particularly when evaluated by advanced statistics.^[17] A notable stretch of games in January saw an injury to Allen necessitate Montembeault taking over as starter again, playing eight straight games and recording a 4-4-0 record with a .930 save percentage in that span. The team's general manager, Kent Hughes, praised his performance and said "he still has a lot to experience as a goalie, but we see the potential. So Sam definitely isn’t going anywhere."^[18] Discussion of whether Montembeault should be treated as the team's main goaltender on a long-term basis began in sports media.^[19] Upon Allen's return, the two largely alternated starts on an equal basis.^[20] Montembeault finished second to team captain Nick Suzuki for the Molson Cup as team player of the year.^[21] Upon the conclusion of the season, he was given the chance to represent Canada internationally for the first time.^[22]

The Canadiens began the 2023–24 season with three goaltenders on the roster, as, in addition to Montembeault and Allen, prospect goaltender Cayden Primeau was no longer exempt from waivers and thus could not be sent down to the AHL's Laval Rocket without possibly being lost to another team. This precipitated discussions about one of the three being traded.^[23] Montembeault appeared in ten games in the first two months of the season, recording a 5-3-1 record and a .910 save percentage, before it was announced on December 1 that he had been re-signed to a three-year, $9.45 million contract.^[24]