2008-09: Austen Brassard was chosen in the first round, 20th overall, of the 2009 OHL Priority Selection by the Windsor Spitfires.
2009-10: In his OHL rookie season, Brassard began the season with the Spitfires. In 37 games for that club, he scored 4 goals and added 8 assists for 12 points. He was dealt to the Belleville Bulls, appearing in 26 games for that team. He scored 6 goals and added 11 assists for 17 points. Brassard was named to the OHL Second All-Rookie Team.
2010-11: In 67 games for the Bulls, Brassard scored 19 goals and added 15 assists for 34 points to go along with 78 penalty minutes. In 4 playoff games, he scored 1 goal. Brassard played for Canada at the 2011 U-18 World Junior Championship, picking up 1 assist in 7 games.
2011-12: Brassard attended Winnipeg's training camp before returning to the Belleville Bulls for his third junior season. He provided both toughness and scoring playing on the Bulls' top line – finishing second on the team in points (51) and third in penalty minutes (71). He scored a career-high 27 goals with 24 assists and was plus-six in 64 games. Belleville reached the playoffs after finishing second in the East Division. Brassard had 1 goal with 1 assist and was minus-two with 6 penalty minutes in the six-game playoff series with the Ottawa 67's.