Growing up in Ontario, Mascherin played with the Georgetown Raiders of the
Ontario Junior Hockey League and Vaughan Kings Minor Midget team of the
Greater Toronto Hockey League (GTHL). During the 2013–14 season, Mascherin won the GTHL's Player of the Year Award after recording 67 goals and 62 assists in 61 games. Mascherin said he tried to model his game after
Tampa Bay Lightning star
Steven Stamkos.
Major junior Following the 2013–14 season, Mascherin was drafted second overall by the
Kitchener Rangers in the 2014
Ontario Hockey League (OHL) Priority Selection. The Rangers' head coach Troy Smith also praised his shooting ability, saying "Mascherin has a world-class shot with a world-class release." He subsequently committed to join the Rangers for their
2014–15 season. Prior to the start of the season, Mascherin was invited to participate in Canada's national under-17 development camp and was eventually named to the Team Canada Red roster for the
2014 World U-17 Hockey Challenge. Coach Troy Smith and his coaching staff moved Mascherin from his natural centre position to the left wing so he could work on creating offence. which earned him the team's Michael Pappert Memorial Trophy as Rookie of the Year and a selection to the OHL's Second All-Rookie Team. Following his rookie season, Mascherin experienced a breakout sophomore campaign with points in each of his first eleven games. He was injured in his 12th game of the season but returned to the lineup after a few games and continued to register points. By December, Mascherin was second in team scoring with 14 goals and 20 assists and he finished the season with a total of 81 points in 61 games. It was later debated amongst scouts that Mascherin was only ranked so low due to his height. Upon finishing the
2015–16 season with 35 goals and 81 points in 65 games, Mascherin was drafted by the
Florida Panthers in the second round of the 2016 NHL Entry Draft. Prior to being drafted, Panthers general manager
Tom Rowe described Mascherin as "the best pure goal scorer in the OHL, if not all of Canada." and Rookie Tournament. Prior to returning the Rangers for the
2016–17 season, Mascherin said he would eagerly work on what the coaching staff told him to. On January 2, 2018, Mascherin scored a career-high four goals in one game as the Rangers beat the
Peterborough Petes in a 5–4 overtime win. He was eventually drafted in the fourth round, 100th overall, by the
Dallas Stars and signed a three-year
entry-level contract on July 13. Following the draft, Stars General Managaer
Jim Nill said "[t]his is his third, fourth year of junior. We all knew about him. He's been drafted. We knew about the situation...He's a good player and the nice thing is we can turn him pro right away, put him in Austin and go from there." After attending the Stars' development camp, Mascherin was reassigned to their
American Hockey League (AHL) affiliate, the
Texas Stars. While playing with the team, he recorded his first professional goal on October 12, 2018, in a 4–3 shootout win against the
Iowa Wild. On November 25, Mascherin recorded a first period
hat-trick in an eventual 7–3 win over the
San Antonio Rampage. Following the hat trick game, he was moved up to the top line with
Denis Gurianov and
Justin Dowling. Mascherin was limited to only 30 games during the
2019–20 season, finishing with four goals and seven points. After attending the Stars' training camp prior to the
2020–21 season, Mascherin was reassigned to the AHL. Upon re-joining the team, Mascherin recorded 11 goals and 23 points by March and was tied for second in the league in points and goals. ==Career statistics==