2007-08: Chosen ninth overall by the Plymouth Whalers in the 2008 OHL Priority. Seguin played for the Toronto Young Nationals of the GTHL in 2007-08, scoring 39 goals and adding 47 assists for 86 points in 51 games.
2008-09: Seguin’s OHL rookie season was a successful one. In 61 games for the Plymouth Whalers, he scored 21 goals and added 46 assists for 67 points. He was +14 for the season and added 28 penalty minutes. Seguin continued his strong play in the 2009 OHL playoffs, scoring 5 goals and adding 11 assists for 16 points in 11 post-season games for the Whalers.
2009-10: In 63 games for the Whalers, Seguin scored 48 goals and chipped in 58 assists for 106 points, tying him for the first overall in OHL scoring with Taylor Hall. In 9 playoff games, Seguin scored 5 goals and added 5 assists for 10 points. Seguin won the Red Tilson Trophy as the OHL’s Most Outstanding Player. He played for Team Orr at the 2010 CHL Top Prospects Game, and also played in the Canada-Russia Challenge and the OHL All-Star Game.