He formerly played for Chinnor Rugby Club and Saracens F.C. Seymour joined Saracens on a full-time academy contract at the start of the 2004/05 season. But after only a few weeks of pre-season training, he was promoted to the first team squad and made his first full appearance as a replacement for Richard Hill.
He established himself as a regular in the first team and made his debut at England U21 level in both the Six Nations and World Championships in Argentina. Seymour was called up to the England Saxons squad for the Churchill Cup in Canada.
Seymour started 13 games for Saracens in the 2007-2008 season and appeared as a replacement 11 times.
A knee injury at the start of the 2008-2009 season restricted his appearances in the early stages of the season. On 26 June 2009, after being released by Saracens, Sale Sharks announced that they had signed Seymour on their official website.