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. Lemieux was chosen in the second round, 31st overall, of the 2014 NHL Draft by the Buffalo Sabres.
Lemieux was part of the deal that sent Tyler Myers, Drew Stafford, Joel Armia and a 2015 first round pick from the Buffalo Sabres to the Winnipeg Jets in exchange for Evander Kane, Zach Bogosian, and Jason Kasdorf.