2010-11: Brendan Lemieux skated in the Toronto Red Wings bantam program after playing youth hockey in Arizona.
2011-12: Lemieux was a team captain for the Toronto Red Wings' midget AAA team. In 26 regular season games he scored 9 goals with 23 assists. He committed to playing college hockey at North Dakota in 2014-15 in February 2012 and signed a tender offer with the USHL's Green Bay Gamblers for the 2012-13 season. He was selected by Barrie in the fourth round (69th overall) of the 2012 OHL Priority Selection.
2012-13: Lemieux began the year with Green Bay before leaving the Gamblers in November to play for Barrie in the OHL. He scored 1 goal with 1 assist and was -2 with 34 penalty minutes in 11 USHL games. Lemieux played 42 regular season games for Barrie — scoring 6 goals with 8 assists and finishing +2 with 52 penalty minutes. The Colts finished first in the Central Division and reached the OHL finals against London. Lemieux scored 2 goals and was +3 with 36 penalty minutes in 21 playoff games.
2013-14 Lemieux skated for Canada's gold medal-winning U18 team at the 2013 Ivan Hlinka Memorial Tournament in August before returning to Barrie for his second OHL season. He scored 1 goal with 1 assist and was +1 with 4 assists in five games at the Ivan Hlinka tournament. Lemieux was selected to play for Team Orr in the 2014 CHL/NHL Top Prospects Game.