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 skated in 65 games for the Ottawa 67’s in his first full OHL season. He scored 2 goals with 21 assists, trailing only Nevin Guy in scoring amongst Ottawa defensemen, and was minus-21 with 64 penalty minutes. The season was a struggle for the 67’s who finished last in the East Division and missed the playoffs for the second straight year. Middleton skated for Team Orr in he CHL/NHL Top Prospects Game as an injury replacement for Saskatoon’s Nelson Nogier. He was ranked 172nd among North American skaters in the Central Scouting final rankings and was selected by Los Angeles in the seventh round (210th overall) in the 2014 NHL Draft.
2014-15: Middleton attended his first NHL training camp with Los Angeles before returning to Ottawa for his second full season with the 67s. Playing in the top two pairings much of the year, he was a big part of Ottawa’s turnaround season. Middleton scored 4 goals with 23 assists and was +11 with 62 penalty minutes in 64 regular season games. The 67’s finished second in the East Division, falling to Niagara in a first-round playoff series. Middleton scored 1 goal with 1 assist and was -5 with 4 penalty minutes in six playoff games.
2015-16: Middleton returned to Ottawa for his fourth OHL season after attending his second training camp with Los Angeles; making his professional debut with the ECHL’s Manchester Monarchs following the OHL playoffs. He had 1 assist and was +1 in two regular season games with the Monarchs. A team captain for the 67’s, Middleton chipped in offensively and was among the team leaders in penalty minutes. He scored 7 goals with 24 assists and was +11 with 68 penalty minutes in 68 regular season games. Ottawa finished second to Kingston in the East Division and lost to Niagara in a first-round playoff series. Middleton had 2 assists and was -3 with 2 penalty minutes in five playoff games.
Middleton entered the 2013-14 season as one of the more highly-regarded defense prospects but was not selected until the final pick of the 2014 NHL Draft after enduring a difficult season with the Ottawa 67's. The second of two Ottawa defensemen selected by the Kings in that draft (Alex Lintuniemi was the King's second round pick), Middleton is more defensive-oriented than Lintuniemi while using his significant size to play a punishing style. He does not have an offensive skating game and seems uncomfortable with the puck at times — though he can contribute on the transition and has a strong first pass. When he forces the play he sometimes gets himself in trouble. Middleton is a tough fighter and a willing combatant when the situation arises. He continues to develop his positional play so as to maximize the effect of his physical presence.
Middleton skated for the Ottawa 67's and was a team captain in 2015-16 in his fourth season of major junior hockey. He was not signed to an entry-level contract by Los Angeles, becoming an unrestricted free agent. Middleton projects as a middle pairing defender who plays a punishing style.