2010-11: Nick Ritchie appeared in a junior game as a 15-year-old — skating with the OJHL's Georgetown Raiders — and was one of the top forwards for the Toronto Marlboros minor midget AAA team. Ritchie scored 50 goals with 45 assists and had 119 penalty minutes in 68 games. The Marlboros competed in the OHL Cup and Ritchie scored 5 goals with 4 assists and 10 penalty minutes in seven games. He played for Team Ontario in the Canada Winter Games; scoring 2 goals with 2 assists and 38 penalty minutes in six games. Ritchie was the second player taken in the OHL Priority Draft when he was selected in the first round by Peterborough.
2011-12: Ritchie was one of the youngest players in the OHL; skating in 62 games for the Peterborough Petes in his rookie season. He scored 16 goals with 23 assists and was -22 with 60 penalty minutes. Peterborough finishing fourth in the East Division; missing the playoffs. Ritchie played for third-place Canada Ontario in the 2012 U17 World Hockey Challenge; scoring 1 goal with 2 assists and 2 penalty minutes in three games.
2012-13: Ritchie was the third-leading scorer for Peterborough in his second OHL season and played for Canada's U18 team in a pair of tournaments. Despite an early season shoulder injury that forced him to miss most of the first two months, he scored 18 goals, one behind co-leaders Brett Findlay and Stephen Pierog, and had 17 assists in 41 games and was -10 with 50 penalty minutes. Peterborough had the OHL's third-worst record and missed the playoffs after finishing fourth in the East Division. Ritchie won a pair of gold medals with Canada, scoring 1 goal with 1 assist and finishing +2 with 31 penalty minutes in five games at the 2012 Ivan Hlinka Memorial Tournament and scoring 1 goal with 3 assists in four games at the 2013 U18 World Junior Championship.
2013-14: Ritchie attended Canada' U18 team summer camp before returning to Peterborough for his third season and was named an assistant captain for the Petes.