Frederic was born at Stainton le Vale in Lincolnshire on 19 September 1868, the son of Francis Hugh Deane, Rector of Horsington and Stainton, and his wife and 2nd cousin, Emma Anne, the daughter of Robert Micklem Deane of Caversham in Oxfordshire (now Berkshire).
On 25 August 1869, Seymour had married Agnes Austin, the eldest daughter of Rev. H. D. Wickham of Horsington, Somerset and they had three daughters and one son, Albert Victor Francis Seymour, who was born when Seymour was 74 years old and later served as a Page of Honour to Queen Victoria.