He was born at the Clarendon Hotel, London. Seton may have been so named following his father being created
Earl of Winton, a title previously held by the
Seton family. His mother, Theresa Newcomen, born in
Calcutta, was an illegitimate daughter of
Thomas Gleadowe-Newcomen, 2nd Viscount Newcomen. Lady Theresa died in December 1853 and Seton's father remarried in 1858, his new wife being Lady Adela Caroline Harriet Capell (1828–1860), daughter of
Arthur Capell, 6th Earl of Essex. Seton had two half-sisters through this marriage: • Lady Sybil Amelia Montgomerie (died 3 February 1932) • Lady Hilda Rose Montgomerie (died 18 June 1928) Seton's older sister, Lady Egida Montgomerie (died 13 January 1880), married
Frederick William Brook Thellusson, 5th Baron Rendlesham. His older brother was
Archibald Montgomerie, who became the 14th Earl of Eglinton (3 December 1841 – 30 August 1892). Seton had the rank of Lieutenant in the service of the
Scots Fusilier Guards and married Nina Janet Bronwen Peers Williams, daughter of Lt.-Col.
Thomas Peers Williams, on 11 June 1870. Seton died in
Windsor, aged 37, from the effects of diabetes. He would have inherited the title
Earl of Eglinton if he had not died so young, and having only daughters who at the time were unable to inherit the property and title, his younger brother
George (1848–1919) became the 15th Earl and George's son
Archibald became the 16th Earl of Eglinton and Winton. ==Offspring==