The elder brother of Michael Hobart Seymour, he was the son of John Crossley Seymour, vicar of Caherelly in the Church of Ireland diocese of Cashel, who married Catherine Wight, the eldest daughter of reverend Edward Wight, rector of Meelick, County Limerick, a member of an old Surrey family.
He was drawn in early life to the religious group formed by Selina Hastings, Countess of Huntingdon.