2009-10: Drafted by London in the fourth round (65th overall) of the 2009 OHL Priority Draft, Griffith appeared in 17 regular season games and 10 playoff contests for the Knights. He spent the majority of the season with the St. Mary's Lincolns in the Greater Ontario Junior Hockey League. Griffith scored 2 goals with 1 assist and was minus-five during the regular season for London, which had the OHL's third-best record. In the playoffs he scored 4 goals with 3 assists and was plus-one with 2 penalty minutes. Griffith was the leading score for St. Mary's, scoring 43 goals with 35 assists and had 56 penalty minutes in 49 games. After finishing fourth in the Western Conference St. Mary's lost to LaSalle in the first round. In five playoff games Griffith scored 6 goals with 3 assists.
2010-11: Griffith was the third-leading scorer for London in his first full OHL season and represented Canada at the 2011 U18 World Junior Championship. In 68 games he scored 22 goals with 40 assists and was -1 with 28 penalty minutes. The Knights reached the OHL playoffs despite finishing last in the tough Midwest Division. In the six game series with eventual OHL champion Owen Sound, Griffith was the Knights' leading scorer with 3 goals and 4 assists and was minus-two with 6 penalty minutes. Griffith was scoreless with an even plus/minus in seven games for fourth-place Canada at the WJC. He was ranked 157th amongst North American prospects in Central Scouting's final rankings prior to the 2011 NHL Draft but was not selected.
2011-12: Griffith returned to London for a second season after attending the Tampa Bay Lightning training camp. He was the Knights' leading scorer with 45 goals and 40 assists and was plus-24 with 49 penalty minutes in 68 games. London finished with the OHL's best record, winning the Midwest Division, and advanced to the Memorial Cup by capturing the playoff title. Griffith had an outstanding post-season, scoring 10 goals with 13 assists while finishing plus-14 in 19 OHL playoff games. In the Memorial Cup he scored 1 goal with 2 assists and was minus-three with 4 penalty minutes. Shawinigan defeated London, 2-1 in overtime, in the Memorial Cup championship game. Griffith was not among the 210 North American skaters in Central Scouting's final ranking yet was selected by Boston in the fifth round (131st overall) in the 2012 NHL Draft.
Seth Griffith is a small, skilled forward with excellent hockey sense and solid offensive skills. The overager out of Ontario broke out in a big way this season, doubling his previous high in goals en route to an OHL Championship with the London Knights. The big difference in Griffith’s game was an impressive compete level that saw him winning pucks from much bigger defenseman all through the playoffs. Offensively, Griffith’s best attribute is his hockey sense. He thinks the game at a high level and knows how to get the most out of his linemates. Griffith’s potential is somewhere between an offensively inclined third liner and a complementary second liner.