2009-10: Mitchell Heard opened the season with the OHL's Plymouth Whalers, making the club as a free agent, before being re-assigned to the Junior A Bowmanville Eagles in November. In 16 games with the Whalers he scored 2 goals with 1 assist and was an even plus/minus with 4 penalty minutes. He did not appear in any playoff games for Plymouth, which finished second in the West Division and reached the second round. In 22 games for Bowmanville he scored 17 goals with 13 assists . The Eagles reached the CCHL playoff finals against Newmarket after finishing first in the East Division. Heard and Michael Markovic shared the team lead in playoff points; each scoring 6 goals with 13 assists in 22 games.
2010-11: Heard skated in 66 games in his first full OHL season with Plymouth. He scored 19 goals with 29 assists and was plus-four with 67 penalty minutes. Plymouth finished third in the West Division and reached the second round of the playoffs. Heard scored 1 goal with 2 assists and was plus-one with 10 penalty minutes in 11 playoff games. He was eligible for the NHL Draft for the second time but was not among the North American skaters listed in Central Scouting's final rankings and was not selected.
2011-12: Heard was a point-per-game scorer while finishing second in penalty minutes for Plymouth as the Whalers finished first in the West Division. In 57 games he scored 29 goals with 28 assists and was plus-six with 111 penalty minutes (trailing only Capitals' prospect Tom Wilson (141)). After finishing two points behind OHL champion London in the regular season, Plymouth reached the second round of the playoffs – falling to Kitchener in seven games. In 13 playoff games Heard scored 4 goals with 7 assists and was an even plus/minus with 26 penalty minutes. Not listed in Central Scouting's final rankings prior to the 2012 NHL Draft, he was selected by Colorado in the second round (41st overall).
2012-13: Heard made his pro debut as a 20-year-old, opening the season with Colorado AHL affiliate Lake Erie and playing 23 games before being returned to juniors in January. He scored 1 goal with 3 assists for the Monsters and was -5 with 72 penalty minutes. After re-joining Plymouth as an over-age player, he scored 17 goals with 19 assists and was +14 with 34 penalty minutes in 32 games. The Whalers finished first in the OHL's West Division and reached the Western Conference finals. Heard scored 8 goals with 5 assists in 14 playoff games and was +8 with 37 penalty minutes.