2010-11: Michael Amadio played for the Northern Ontario champion Soo Greyhounds bantam AAA team in his hometown. He scored 60 goals with 74 assists and 16 penalty minutes in 39 regular season games and had 10 goals with 9 assists and 2 penalty minutes in six playoff games. He led the Greyhounds with 9 goals and 4 assists in eight games during the provincial tournament.
2011-12: Amadio was the leading scorer for the Sault Ste. Marie Northstars midget team. In 29 regular season games he scored 32 goals with 30 assists and 12 penalty minutes. Amadio scored 5 goals with 9 assists and 6 penalty minutes in 11 playoff games. He played for the NOHA Wolves in the 2012 OHL Cup — scoring 5 goals with 4 assists in nine games — and was selected by the Brampton Battalion in the second round (36th overall) of the 2012 OHL Priority Draft.
2012-13: Amadio skated in 63 of 68 games for Brampton as a 16-year-old OHL rookie. He scored 6 goals with 13 assists and was -3 with 8 penalty minutes. The Battalion finished second in the Central Division behind Barrie. Amadio was -1 with no points nor penalties in Brampton's five-game playoff series with Sudbury.
2013-14: Amadio was the sixth-leading scorer for the re-located Battalion, which moved from Brampton to North Bay. In 64 regular season games he scored 12 goals with 26 assists and was +15 with 14 penalty minutes. Brampton finished first in the Central Division. Amadio was ranked 56th amongst North American skaters in Central Scouting's mid-term rankings prior to the 2014 NHL Draft.