2009-10: Drafted by Kingston in the third round (42nd overall) of the 2009 OHL Priority Draft, Clark Seymour played in 60 games for the Frontenacs as a 16-year-old rookie. He scored 1 goal with 5 assists and was minus-11 with 58 penalty minutes. Kingston finished second in the East Division and lost to Brampton in a seven-game first round playoff series. In five playoff games Seymour was minus-2 with 2 penalty minutes.
2010-11: Seymour was obtained by the Peterborough Petes in a multi-player November trade after opening the season with Kingston. In 18 games with the Frontenacs he scored 2 goals with 2 assists and was plus-three with 14 penalty minutes. In 40 games with Peterborough he scored 1 goal with 2 assists and was minus-14 with 45 penalty minutes. The Petes had the OHL's second-worst record and failed to reach the playoffs.
2011-12: Seymour skated in 47 of 68 games for Peterborough in his first full season with the Petes – missing time due to a shoulder injury late in the season. Playing on a Petes team that missed the OHL playoffs for the second straight season, he was minus-four with 8 assists and had 96 penalty minutes. In his second year of draft eligibility, Seymour was selected by Pittsburgh in the fifth round (143rd overall) of the 2012 NHL Draft.
Clark Seymour is a big, physical defenseman who prides himself in being hard to play against and tough in the corners. His game is very pro-ready in the sense that he already plays a very simple, stay-at-home style. He is ferociously physical in his own end, demonstrating a great willingness to throw his massive frame around, and is not afraid to drop the gloves to stick up for teammates. Many parts of his game are fairly sophisticated for a junior level player as he normally keeps his head up and is competent at head fakes, and making reads. Still, he will never be regarded as an offensive, or even two-way defenseman.
In terms of bringing his game to the next level, Seymour needs to play more consistent, and play with a greater deal of confidence. Though he is relatively mobile given his massive frame, he could still stand up improve his footspeed.
Seymour will return to the OHL for the 2012-13 season. He projects as a depth shutdown defenseman at the professional level.