2010-11: Aaron Ekblad skated for the Sun County Panthers midget minor team as a 14-year-old. In 48 midget major games – including 18 playoff contests – he scored 9 goals with 46 assists and had 48 penalty minutes. Sun County captured the Alliance championship. Ekblad was selected first overall by Barrie in the 2011 OHL Priority Draft after being granted exceptional player status by Hockey Canada.
2011-12: Ekblad was the leading scorer amongst defensemen for Barrie in his first OHL season and played for Canada Ontario in the 2012 U17 World Hockey Challenge. He scored 10 goals with 19 assists in 63 OHL games and was -5 with 34 penalty minutes. The Colts were second in the Central Division and reached the second round in the playoffs one year after finishing with the league's worst record. Ekblad scored 2 goals with 3 assists and was -1 with 8 penalty minutes in 13 playoff games. In five games at the WHC he scored 1 goal with 2 assists and 4 penalty minutes as Canada Ontario won the bronze medal.
2012-13: Ekblad was the second-leading scorer amongst Barrie defensemen behind overage 20-year-old Ryan O'Connor in his second season with the Colts. He scored 7 goals with 27 assists and was +29 with 64 penalty minutes in 54 games; playing in just two games in December due to a lower body injury. The Colts finished first in the Central Division and reached the OHL Finals against London. Ekblad scored 7 goals with 10 assists and was +11 with 28 penalty minutes in 22 playoff games. He was the captain of Canada Ontario in the 2013 U17 World Hockey Challenge; scoring 2 goals with 4 assists and 4 penalty minutes in five games.
2013-14: Ekblad returned to Barrie for his third OHL season after serving as captain for Canada's gold medal-winning U18 team at the Ivan Hlinka Memorial Tournament in August. In five games at the Ivan Hlinka tournament he was +4 with 2 goals, 2 assists and 4 penalty minutes.