The NHL's 1967 expansion offered an opportunity to play for the newly founded North Stars, for whom he scored the first goal in the franchise's history.

[1][2] He added eight points in six games during the Memorial Cup playdowns, however St. Boniface failed to reach the national championship final.

[3] Masterton played three seasons in Denver between 1958 and 1961, appearing in a total of 89 games, scoring 66 goals and 130 assists for 196 points in that time.

[5] He was a two-time NCAA All-American and was twice named to the Western Collegiate Hockey Association (WCHA) All-Star team, earning both awards in 1960 and 1961.

[7] Masterton served as team captain for 1960–61,[8] and was named the most outstanding player of the 1961 national championship as he led Denver to a second consecutive title.

[9] The Pioneers finished the season with a 30–1–1 record and were hailed as "the greatest hockey team to ever represent an American college or university.

"[10] Turning to professional ice hockey after graduating with an engineering degree,[9] Masterton signed a contract with the Montreal Canadiens in 1961.

[3] Masterton was promoted to the Cleveland Barons of the American Hockey League (AHL) for the 1962–63 season, where he led the team with 82 points.

[11] He finished as the runner up to Doug Robinson for the Dudley "Red" Garrett Memorial Award as the AHL's top rookie.

[12] Faced with little opportunity to make the Montreal roster,[3] Masterton left the professional game to complete his master's degree at the University of Denver.

[3] At the age of 29, Masterton made his NHL debut in the North Stars' inaugural game, played on October 11, 1967, against the St. Louis Blues.

[14] His wife Carol later recalled that it was a "dream come true" for her husband: "He always wanted a shot at the NHL, and expansion was a wonderful thing for him and guys like him.

[11] Masterton suffered a severe internal brain injury during the first period of Minnesota's January 13, 1968, game against the Oakland Seals at the Met Center.

[19] He carried the puck up the ice at full speed, passing it off as two Seals defencemen, Larry Cahan and Ron Harris, converged on him.

[11] Referee Wally Harris compared the hit to an explosion, adding "he was checked hard, but I'm sure it wasn't a dirty play.

[16] Legislators in New York considered a law to make their use mandatory,[21] and the NHL voted on and rejected a rule requiring players wear helmets three times by 1971.

Goaltender Cesare Maniago recalled that the night before the fatal hit, Masterton had been complaining of severe migraines that he had had for over a week.