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.
His first work was Vital Christianity, a series of letters on religion, addressed to young persons; it appeared in 1810; a second edition was published in 1819.
In 1816 Seymour published a memoir of Charlotte Brooke, prefixed to an edition of her Reliques of Irish Poetry.
[2] Interested in hymnology, he assisted Josiah Miller in preparing his Singers and Songs of the Church.