2010-11: Mitchell Dempsey played for the Cambridge Hawks minor midget AAA team in Suburban Toronto. In 29 games he scored 17 goals with 27 assists and 38 penalty minutes. In seven playoff games he had 6 goals and 8 penalty minutes. He was selected by Plymouth in the first round (11th overall) of the 2011 OHL Priority Draft.
2011-12: Dempsey played 34 games for Plymouth in his first OHL season and represented Canada Ontario in the 2012 U17 World Hockey Challenge. He scored 1 goal with 4 assists and was -8 with 29 penalty minutes for the Whalers. Plymouth finished first in the West Division and reached the second round in the playoffs. Dempsey played in five playoff games and had an even plus/minus with no points nor penalty minutes. In six games at the WHC he scored 2 goals and had 12 penalty minutes as Canada Ontario won a bronze medal.
2012-13: Dempsey was part of an August trade between Plymouth and Sault Ste. Marie. He skated in 36 games for the Greyhounds, missing time due a case of pneumonia and a nagging ankle injury. Dempsey scored 1 goal with 4 assists and was an even plus/minus with 17 penalty minutes. Sault Ste. Marie finished second to Plymouth in the West. Dempsey didn't play in the six-game playoff series with Owen Sound. Not among the 210 North American skaters in Central Scouting's final rankings, Dempsey, the nephew of long-time Bruins defenseman and assistant general manager Don Sweeney, was drafted by Boston in the seventh round (210th overall) in the 2013 NHL Draft.
Dempsey is a rugged forward who is strong on the puck, but has yet to develop a scoring touch at the OHL level.
Dempsey return to the OHL for the 2013-14 season.