He was the elder brother of Sir Francis Crossley (1817–1872), who had served as the constituency's MP from 1852 to 1859.
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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 the eldest daughter of Edward Wight, rector of Meelick, County Limerick, a member of an old Surrey family.
He was born on 29 September 1800, the sixth son of John Crossley Seymour, vicar of Caherelly (d. 19 May 1831), who married in January 1789 Catherine, eldest daughter and coheiress of Rev. Edward Wight, rector of Meelick in Limerick.