Seldon's first teaching appointment was at Whitgift School in Croydon in 1983, where he was Head of Politics and taught in the Sixth Form. In 1989 he returned to his old school, Tonbridge, and became Head of History and General Studies. In 1993 he was appointed Deputy Headmaster and, ultimately, Acting Headmaster of St Dunstan's College in London. He then became Headmaster of Brighton College from September 1997 until he joined Wellington College in January 2006 as its 13th Master. He became Executive Principal at The Wellington Academy (a separate school) in 2013.