While in Ässät, Armia was considered one of the most talented players of his age group and a future superstar by media,[2][3] due much in part to his natural goal scoring prowness.

[11] During his second North American professional season in 2014–15, Armia was recalled from Rochester and made his NHL debut with the Sabres against the Detroit Red Wings on 23 December 2014.

[15] The 2017–18 season built upon this personal success, as Armia finished with 12 goals and 29 points in 79 games to establish then career highs across all statistical categories.

[18] On 30 June 2018, the Jets traded Armia along with goaltender Steve Mason and two draft picks to the Montreal Canadiens in exchange for defenceman Simon Bourque.

[21][22] On 6 October, Armia scored his first goal for Montreal, a shorthanded marker that gave the Canadiens a 4–0 lead over the Pittsburgh Penguins en route to a 5–1 victory.

[25] A few days later, Armia converted the decisive goal in the fifth round of a shootout victory against the New York Islanders on 5 November; being the only player from either team to beat the goaltender.

[28][29] On 3 January 2019, Armia returned from injury against the Vancouver Canucks; he was credited with an assist on Jordie Benn's opening goal in a 2–0 Montreal win.

[31] On 11 July 2019, Armia signed a two-year contract worth $5.2 million with the Canadiens having previously filed for salary arbitration as a restricted free agent.

[35] Prior to the beginning of the 2023–24 season, the Canadiens placed Armia on waivers for the purpose of re-assignment to AHL affiliate, the Laval Rocket.

[56] In December 2013, Armia, along with the AHL's Rochester Americans, were selected to participate in the annual Spengler Cup tournament held in Davos, Switzerland.

[59] With the Canadiens failing to qualify for the Stanley Cup playoffs following the 2021–22 NHL season, Armia accepted an invitation to play for the Finnish national senior team for the first time at the 2022 IIHF World Championship, which was held in his home country.

[67] Finland ultimately lost to Canada in the quarterfinals matchup[68] whereas Armia played eight games and recorded two goals and an assist for a total of three points.

[70] Armia was born in Pori, and attended a Swedish-speaking school, the Björneborgs svenska samskola, known for having many talented ice hockey players as students.