2008-09: Derek Mathers was a defenseman for the Elgin-Middlesex Chiefs midget minor team. In 20 games, he scored 2 goals with 5 assists and had 60 PMs. Mathers was selected by the Peterborough Petes in the 4th round (79th overall) of the 2009 OHL Priority Draft.
2009-10: Mathers played for the Strathroy Rockets in the Greater Ontario Junior Hockey League and scored 2 goals with 5 assists and 53 PMs in 43 games.
2010-11: Mathers had a team-leading 171 penalty minutes with Peterborough as an OHL rookie. Playing forward for the Petes, who had the second-worst record in the league and missed the playoffs, Mathers scored 1 goal with 4 assists and was -11 plus/minus. Mathers was ranked 197th amongst North American skaters by Central Scouting in their mid-term rankings but was not among the 210 players in the final rankings. He was selected by the Flyers in the 7th round (206th overall) in the 2011 NHL Draft.
2011-12: Mathers made his pro hockey debut – skating in nine late season games for Flyers' AHL affiliate Adirondack after his season with the Peterborough Petes had ended. He had no points and was minus-two with 26 penalty minutes during his stint with the Phantoms — who missed the AHL playoffs after finishing fifth in the Northeast Division. Mathers was one of the OHL's heavyweights in his second season with Peterborough, finishing third in the league with 177 penalty minutes. He scored 9 goals with 8 assists and was minus-19 in 65 games for a Peterborough team that missed the OHL playoffs; finishing fourth in the East Division.
Mathers is one of the toughest fighters in the OHL and is the prototypical energy forward with the size and strength to fill that role. A defenseman during his youth hockey days, Mathers does possess a big shot and decent hands but his skating and agility will need to improve if he is to make the move into pro hockey. Mathers' work ethic is a plus and he is popular with teammates.
Playing for the Peterborough Petes in 2012-13. Long-term, Mathers' main role at the pro level will appear to be that of a heavyweight enforcer and lower line forward.