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.
2013-14: Dempsey attended training camp with the Bruins before being returned to the Sault Ste. Marie Greyhounds but was released by the OHL club after just two games. He later skated in six games for the Kitchener Rangers but finished the season playing Junior A hockey with the Greater Ontario Junior Hockey League’s Kitchener Dutchmen. Dempsey had 1 assist and was -3 with 10 penalty minutes in eight OHL games. He had 5 assists and 30 penalty minutes in 15 regular season games with the Dutchmen and scored his only goal of the year with 12 penalty minutes in two playoff games.
2014-15:Dempsey joined the QMJHL’s Saint John’s Sea Dogs in 2014-15 after limited success in the OHL in three seasons. In 60 regular season games he scored 8 goals with 17 assists and was -9 with 47 penalty minutes. He was -1 with no points and 4 penalty minutes in three playoff games. Dempsey was not signed to an entry-level contract by Boston, becoming an unrestricted free agent. He was invited to training camp with the Bruins but released.
Dempsey has the prototypical size of a rugged, power forward but doesn't play an overly abrasive game. He is an average skater with limited puck handling, shooting and passing skills. Dempsey is having his most productive season in junior hockey in 2014-15 in his first season in the QMJHL after limited ice time in three OHL seasons but he is still is a long-range project at this point.
Dempsey attended the Providence Bruins' training camp hoping to secure an AHL deal but was released. In November 2015 he was signed to an ECHL tryout contract with the Manchester Monarchs. Dempsey has the prototypical size of a power forward, and these players sometime take longer to develop. Long-range his skill set suggests he can be effective in a lower line role in the NHL one day. He has a significant way to go to reach that potential.