Vancouver Canucks

On March 31, 2018, Demko made his NHL debut for the Canucks, stopping 26 shots en route to a 5–4 overtime victory over the Columbus Blue Jackets.^[16] Demko would spend the rest of the season with the Utica Comets (AHL).

On January 2, 2019, the Canucks announced they had dealt backup goaltender Anders Nilsson to the Ottawa Senators, opening up a full-time backup position for Demko.^[17] Demko, who had spent most of the season recovering from a concussion^[18] and playing in the AHL, would finish the season on the Canucks roster for the first time.

On April 24, 2019, the Canucks signed Demko to a two-year, $2.1 million contract extension.^[19] In the 2019-20 season that followed, Demko would continue playing the backup for the majority of the regular season, until a serious knee injury sidelined the incumbent Jacob Markstrom for the next month.^[20] Beginning on February 25, 2020 against the Montreal Canadiens, Demko would start 7 of the final 8 games of the season, going 3–4–0 and posting a .906 save percentage. His most impressive game in that stretch came on March 10, 2020, a 45-save shootout win over the New York Islanders, which happened to be the last Canucks game before the 2019–20 season was cut short.

On September 1, 2020, Demko would make his first NHL playoff start in relief of the injured Markstrom. With the Canucks facing elimination and down 3–1 in the Western Conference Second Round to the Vegas Golden Knights, Demko allowed just 1 goal on 43 shots in a 2–1 win over Vegas. In doing so, he became the first rookie goaltender since José Théodore in 1997 to win an elimination game in his playoff debut.^[21] On September 3, 2020, he made 48 stops and recorded his first career playoff shutout in a 4–0 victory over Vegas, becoming the first rookie goaltender to win his first two career playoff starts while facing elimination since Jacques Plante in 1953.^[22] The 48-save shutout in game 6 of the series would set an NHL record for most saves in a playoff shutout by a rookie, as well as the most saves by a goaltender in a game that ended in regulation.^[23] In his first three playoff starts, Demko made 123 stops on 125 shots for a remarkable .984 save percentage, nearly leading the Canucks to a 7 game series upset of the top seeded Golden Knights. Demko was named the first star for all three of the games he played.

On March 1, 2021, Demko earned his first career regular season shutout in a 4–0 victory over the Winnipeg Jets.^[24] The shutout would be the first game in an 8–1–0 run by Demko to start the month of March, in which he posted a .950 save percentage and a 1.74 goals against average. The run would establish Demko's position as the number 1 in Vancouver's crease going forward. On March 31, Demko signed a five-year, $25 million contract extension with the Canucks.^[25]

On January 2, 2022, Demko was named the NHL Third Star of the Month for December 2021 after posting a 7–1–0 record, 1.72 goals-against average, and .946 save percentage during the month.^[26][27] On January 13, Demko was named an NHL All-Star for the first time in his career.^[28]