[4] Lajoie signed a three-year, entry-level contract with the Senators on October 3, 2016, and attended their training camp.

He made his NHL debut on October 4, 2018, scoring a goal and an assist in a 4–3 overtime loss to the Chicago Blackhawks.

[8] He spent the rest of the season in Belleville, registering 4 goals and 17 points in 48 games before the AHL suspended play on March 12, 2020 due to the COVID-19 pandemic.

[8][13] After attending the Senators training camp for the pandemic-delayed 2020–21 season, on January 12, 2021, Lajoie was traded by Ottawa to the Carolina Hurricanes in exchange for Clark Bishop.

[18] He and Chatfield made their NHL season debuts on November 30, replacing defencemen Brett Pesce and Tony DeAngelo in the lineup.

[8] Following three seasons with the Hurricanes organization, Lajoie left as an unrestricted free agent and agreed to a one-year, two-way contract with the Toronto Maple Leafs on July 1, 2023.

After going unclaimed Lajoie was assigned to the Maple Leafs AHL affiliate, the Toronto Marlies to start the 2023–24 season.

[25][26] He was recalled by the Maple Leafs on November 4 after an injury to Timothy Liljegren and made his season debut against the Buffalo Sabres that night.

[29] An unrestricted free agent in the offseason, Lajoie signed a one-year, two-way contract with the Seattle Kraken on July 1, 2024.

[29] After going unclaimed on waivers, he was assigned to Seattle's AHL affiliate, the Coachella Valley Firebirds, to start the 2024–25 season.