MacKellar was ranked as the 19th best player in the Canadian Football League's Amateur Scouting Bureau final rankings for players eligible in the 2022 CFL Draft.[4] He was then drafted in the first round, sixth overall, in the draft by the Toronto Argonauts.[5] He then signed with the team on May 10, 2022.[5] Following training camp, he was placed on the injured list, but soon after played in his first career professional game on July 4, 2022, against the Winnipeg Blue Bombers.[3] His first game played outside of Toronto's BMO Field was in Wolfville, Nova Scotia as part of Touchdown Atlantic where he dressed as a backup offensive lineman in his home province.[6][7] He played in 16 regular season games in 2022 and started in 12 of them.[8]
In the 2023 season, MacKellar played in all 18 regular season games, starting in one.[8]