2008-09: Zack Mitchell played for the Toronto Marlboros minor midget AAA team in the Greater Toronto league. He scored 42 goals with 48 assists and 74 penalty minutes in 77 games. Mitchell scored 4 goals with 5 assists and 8 penalty minutes for the Marlboros in seven games at the OHL Cup. He was selected by Guelph in the fourth round (71st overall) of the 2009 OHL Priority Draft.
2009-10: Originally committed to playing college hockey at Harvard, Mitchell instead joined the Guelph Storm and played in 64 games, including five playoff contests, in his first OHL season. He played for Canada Ontario in the 2010 U17 World Hockey Challenge. Mitchell scored 3 goals with 7 assists and was -15 with 25 penalty minutes in 59 regular season games as the Storm finished third in the OHL's Midwest Division. He played in all five games in the first round series with London, scoring 2 goals, and was +1 with no penalty minutes. Mitchell scored 1 goal with 2 penalty minutes in six games for silver medal-winning Canada Ontario.
2010-11: Mitchell skated in 61 of 68 regular season games for Guelph in his second OHL season. He scored 9 goals with 10 assists and was -20 with 24 penalty minutes. The Storm finished fourth in the Midwest Division and lost to Saginaw in a six-game first-round playoff series. Mitchell scored 1 goal with 6 assists and was +3 with 2 penalty minutes in six playoff games.
2011-12: Mitchell had a breakout season in his third year with Guelph, leading the Storm in goals (37) and assists (38). Playing in 67 of 68 regular season games he finished +6 with 32 penalty minutes as the Storm finished fourth in the Midwest Division. In Guelph's six-game first round playoff series with Plymouth he scored 2 goals with 2 assists and was -4 with 2 penalty minutes.
2012-13: Mitchell attended the Winnipeg Jets development camp in the summer before returning to Guelph for his fourth season. An alternate captain, he skated in all 68 regular season games and was the Storm's third-leading scorer with 22 goals and 34 assists; finishing +7 with 34 penalty minutes. Guelph finished fourth in the Midwest Division. Mitchell scored 1 goal with 1 assist and was -1 with 4 penalty minutes in the five-game playoff series with Kitchener.
2013-14: Mitchell was invited to Carolina's training camp but was not signed; returning to Guelph for an overage season. He was the Storm's fourth-leading scorer as Guelph had the OHL's best record in finishing first in the Midwest Division. An alternate captain, Mitchell scored 31 goals with 52 assists and was +52 with 40 penalty minutes in 67 regular season games. The Wild signed Mitchell to a three-year, entry-level contract on March 4th, 2014.
The Wild signed Mitchell to a three-year entry-level contract on March 4th, 2014.