The titles of Earl of Hertford and Marquess of Hertford have been created several times in the peerages of England and Great Britain.
The third Earldom of Hertford was created in 1559 for Edward Seymour, who was simultaneously created Baron Beauchamp of Hache. His grandson William Seymour was subsequently created Marquess of Hertford and restored to the title of Duke of Somerset; the Marquessate became extinct in 1675 and the other three titles in 1750.
The present Marquessate was created in 1793. Lord Hertford holds the subsidiary titles of Earl of Yarmouth (Peerage of Great Britain, 1793), Earl of Hertford (Peerage of Great Britain, 1750), Viscount Beauchamp (Peerage of Great Britain, 1750), Baron Conway, of Ragley in the County of Warwick (Peerage of England, 1703), and Baron Conway of Killultagh, of Killultagh in the County of Antrim (Peerage of Ireland, 1712). Lord Hertford's heir uses the style Earl of Yarmouth.
The Marquesses of Hertford are members of the Seymour family headed by the Duke of Somerset. Francis Seymour (1679–1732) was the fourth son of Sir Edward Seymour of Berry Pomeroy, 4th Baronet, a descendant of Edward Seymour, 1st Duke of Somerset (Sir Edward's grandson Sir Edward Seymour, 6th Baronet, of Berry Pomeroy succeeded as 8th Duke of Somerset in 1750). Upon the death of his elder brother, Francis succeeded to the estates of his relative Edward Conway, 1st Earl of Conway. In 1703 he was created Baron Conway in the Peerage of England and assumed the additional surname of Conway. In 1712 he was created Baron Conway of Killultagh in the Peerage of Ireland. In 1750 his son Francis Seymour-Conway, 2nd Baron Conway, was created Viscount Beauchamp and Earl of Hertford. These were revivals of titles previously held by the Dukes of Somerset, which had become extinct the same year on the death of Seymour-Conway's kinsman Algernon Seymour, 7th Duke of Somerset. In 1793 he was further honoured when he was made Earl of Yarmouth and Marquess of Hertford. The latter title had also previously been held by the Dukes of Somerset, but had become extinct in 1675 (see below). As a descendant of Edward Seymour, 1st Duke of Somerset, the present Marquess of Hertford is in remainder to the Dukedom of Somerset and its subsidiary title the Barony of Seymour.
The family seat is Ragley Hall, near Alcester, Warwickshire.