2010-11: Jacob Middleton made his debut in AAA minor midget hockey as a 15-year-old — skating in five games including one playoff contest with the Huron Perth Lakers — after playing in the Stratford MHA youth program in Ontario. He had no points nor penalty minutes with the Lakers.
2011-12: Middleton played four games with the junior A Stratford Cullitons in addition to playing for the Huron Perth Lakers midget teams. He had three assists in four games with the Cullitons. In two games with Huron Perth's midget major team he scored 1 goal with 4 penalty minutes. Middleton was the top scoring defenseman for the Lakers' midget minor team. He scored 7 goals with 16 assists and had 26 penalty minutes in 25 regular season games. In eight playoff games he scored 1 goal with 7 assists and had 16 penalty minutes. In four games at the OHL Cup he had 1 assist and 6 penalty minutes. Middleton was selected by Owen Sound in the first round (eighth overall) of the 2012 OHL Priority Draft.
2012-13: Middleton began his OHL career with Owen Sound before being obtained by the Ottawa 67's as part of the Cody Ceci (Ottawa) trade. He had 1 assist and was +3 with 7 penalty minutes in 14 games with the Attack; missing seven weeks at the beginning the year due to a high ankle sprain. In 15 games with last-place Ottawa he scored 1 goal with 3 assists and was -16 with 18 penalty minutes. Middleton played for Canada Ontario in the 2013 U17 World Hockey Challenge; scoring 1 goal with 1 assist and 16 penalty minutes in five games.
2013-14: Middleton returned to the Ottawa 67's for his second OHL season.