2011-12: Spencer Watson played for the junior A London Nationals as a 15-year-old and had a huge offensive season with the London Junior Knights Gold minor midget team. He had no points in two regular season games with the Nationals and 3 goals with 1 assist and 2 penalty minutes in seven playoff games. Watson scored 5 goals in five Sutherland Cup games. He scored two goals in his only midget major game with the London Knights. Playing alongside his twin brother Matthew Watson with the Knights Gold team, Spencer Watson scored 55 goals with 30 assists and had 36 penalty minutes in 41 regular season games. In five OHL Cup games he scored 4 goals with 4 assists. He was selected by Kingston in the second round (24th overall) of the 2012 OHL Priority Draft.
2012-13: Watson was named to the OHL's All-Rookie first team following his first season with Kingston and played for Canada Ontario in the 2013 U17 World Hockey Challenge. He scored 23 goals with 20 assists and was an even plus/minus with 18 penalty minutes in 63 regular season games. The Frontenacs finished third in the East Division and were swept by Barrie in a first round playoff series. Watson played in two of four playoff games; finishing -1 and scoring 1 goal with 1 assist. In five games for Canada Ontario at the WHC he scored 5 goals with 2 assists and had 2 penalty minutes.
2013-14: Watson skated for Canada's gold medal U18 team at the 2013 Ivan Hlinka Tournament in August and was the tournament's leading scorer before returning to Kingston for his second OHL season. In five games for Canada at the Ivan Hlinka tournament he scored 4 goals with 6 assists and had 27 penalty minutes. Watson was selected to play for Team Orr in the 2014 CHL/NHL Top Prospects Game.