2009-10: Ian McCoshen, a native of Faribault, MN, attended Shattuck-St. Mary's in his hometown and skated for the school's bantam team. In 58 games he scored 21 goals with 35 assists and had 46 penalty minutes.
2010-11: McCoshen played in the USHL as a 15-year-old, skating in 42 games for the Waterloo Black Hawks. He had 6 assists and was minus-nine with 38 penalty minutes. McCoshen played in two playoff games for Waterloo and was minus-one with no points or penalty minutes. The Saginaw Spirit selected him in the sixth round (112th overall) of the 2011 OHL Priority Draft.
2011-12: McCoshen returned to Waterloo for a second USHL season and represented the USA in the 2011 U19 World Junior A Challenge. In 55 games with the Black Hawks he scored 8 goals with 12 assists and was plus-four with 43 penalty minutes. Waterloo reached the USHL finals against Green Bay after finishing third in the West during the season. In 15 playoff games McCosh scored 4 goals with 3 assists and was plus-four with 6 penalty minutes. In five games at the WJAC he had 1 assist with 4 penalty minutes.
2012-13: McCoshen spent a third season in the USHL with Waterloo and in November committed to playing college hockey at Boston College in 2013-14. In 53 games with the Black Hawks he scored 11 goals with 33 assists – both career highs – and was plus-35 with 48 penalty minutes. Waterloo finished third in the West Division. McCoshen scored 2 goals with 2 assists and was minus-two in the five-game playoff series with Fargo. He was ranked 24th amongst North American skaters in Central Scouting's final rankings prior to the 2013 NHL Draft.