2010-11: Mitch Slattery skated for the Tier 1 bantam team at Shattuck-St. Mary's School in Faribault, MN. In 61 games he scored 14 goals with 55 assists and 54 penalty minutes.
2011-12: Slattery played for the U16 midget team in his second year at Shattuck-St. Mary's. In 54 games he scored 5 goals with 25 assists and 50 penalty minutes. The Sabres advanced to USA Hockey's U16 nationals; falling to the Los Angeles Kings Selects, 4-0, in the quarterfinals.
2012-13: Slattery transferred from Shattuck-St. Mary's to Hill-Murray School in Maplewood, MN and played for Team Northeast in the Upper Midwest Elite HS League in the fall. As a junior at Hill-Murray he scored 25 goals with 33 assists and 17 penalty minutes in 30 games. The Pioneers captured the Section 4AA title and advanced to the state championship game against Edina, falling 4-2. Slattery scored 10 goals with 13 assists and had 52 penalty minutes in 24 games for Team Northeast. He attended USA Hockey's Select 17 Festival in July and was selected by Indiana in the 21st round (301st overall) in the 2013 USHL Entry Draft.
2013-14: Slattery skated in one USHL game for Indiana in March following his senior season at Hill-Murray. He committed to attending St. Cloud State in 2014-15 in October while playing for Team Northeast in the fall elite league. He had no points nor penalties in his only game with the Ice. Slattery was the second-leading scorer for the Pioneers with 20 goals and 32 assists and had 38 penalty minutes in 28 games. Hill-Murray entered the Section 4AA tournament as the top seed but fell to third-seeded Stillwater, 2-1, in the championship game. Slattery scored 20 goals with 15 assists and 48 penalty minutes in 24 games for Team Northeast. He was ranked 81st amongst North American skaters in Central Scouting's final rankings prior to the 2014 NHL Draft.