2010-11: Mason McDonald played goal for the Halifax ACCEL Hawks major bantam team that advanced to the Nova Scotia provincial tournament.
2011-12: McDonald played for the Nova Scotia midget AAA champion Halifax Titans. He had a 2.01 goals against and .916 save percentage in 20 regular season games and had a 2.03 goals against and .932 save percentage in six playoff games. Halifax finished third in the Atlantic Regional tournament as Moncton earned the right to compete in the Telus Cup. McDonald was selected by Acadie-Bathurst in the second round (20th overall) of the 2012 QMJHL Entry Draft.
2012-13: McDonald appeared in 26 QMJHL games with Acadie-Bathurst as a 16-year-old and played for Canada Atlantic in the 2013 U17 World Hockey Challenge. He was 6-8-3 with 1 shutout and had a 4.72 goals against and .885 save percentage in QMJHL play. The Titan finished fourth in the Maritimes Division and lost to Blainville-Boisbriand in a five-game first round playoff series. McDonald played one period as a backup to Jacob Brennan in the playoffs; stopping 12 of 13 shots. In four games at the WHC he finished with a 5.34 goals against and .870 save percentage for Canada Atlantic.
2013-14: McDonald was with the gold medal winning Canada U18 squad at the Ivan Hlinka Memorial Tournament in August before returning to Acadie-Bathurst for his second QMJHL season. He did not appear in any games for Canada as a backup to Julio Billia at the Ivan Hlinka tournament. McDonald was selected to play for Team Orr in the 2014 CHL/NHL Top Prospects Game.