The titles of Earl of Hertford and Marquess of Hertford have been created several times in the peerages of England and Great Britain.

The present Marquessate was created in 1793. Lord Hertford holds the subsidiary titles of Earl of Hertford (Peerage of Great Britain, 1750), Earl of Yarmouth (Peerage of Great Britain, 1793), 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.

Earls of Hertford, First Creation (1135)

• Richard de Clare, 1st Earl of Hertford (1100_1136)
• Gilbert de Clare, 2nd Earl of Hertford (d. 1151)
• Roger de Clare, 3rd Earl of Hertford (d. 1173)
• Richard de Clare, 4th Earl of Hertford (1153-1217)
• Gilbert de Clare, 5th Earl of Hertford, 1st Earl of Gloucester (1180-1230)
• Richard de Clare, 6th Earl of Hertford, 2nd Earl of Gloucester (1222-1262)
• Gilbert de Clare, 7th Earl of Hertford, 3rd Earl of Gloucester (1243-1295)
• Ralph de Monthermer, Earl of Hertford and Gloucester (d. 1325) (acquired Earldoms through marriage to the widow of the 6th Earl, and lost them on her death in 1307)
• Gilbert de Clare, 8th Earl of Hertford, 4th Earl of Gloucester (1291-1314)

Earls of Hertford, Second Creation (1537)

• Edward Seymour, 1st Duke of Somerset, 1st Earl of Hertford (1506-1552), forfeit 1552.

Earls of Hertford, Third Creation (1559)

• Edward Seymour, 1st Earl of Hertford (1539_1621)
• William Seymour, 2nd Earl of Hertford (1587_1660) (became Marquess of Hertford in 1641)

Marquesses of Hertford, First Creation (1641)

• William Seymour, 2nd Duke of Somerset, 1st Marquess of Hertford (1587-1660) (restored to the Dukedom of Somerset in 1660)
• William Seymour, 3rd Duke of Somerset, 2nd Marquess of Hertford (1651-1671)
• John Seymour, 4th Duke of Somerset, 3rd Marquess of Hertford (d. 1675) (Marquessate extinct 1675)

Earls of Hertford, Third Creation (continued)

• Francis Seymour, 5th Duke of Somerset (1658_1678)
• Charles Seymour, 6th Duke of Somerset (1662-1748)
• Algernon Seymour, 7th Duke of Somerset (1684_1750)

Barons Conway (1703)

• Francis Seymour-Conway, 1st Baron Conway (1679_1732)
• Francis Seymour-Conway, 2nd Baron Conway (1718_1794) (created Earl of Hertford in 1750 and Marquess of Hertford in 1793)

Marquesses of Hertford, Second Creation (1793)

• Francis Seymour-Conway, 1st Marquess of Hertford (1718-1794)
• Francis Ingram Seymour-Conway, 2nd Marquess of Hertford (1743-1822)
• Francis Charles Seymour-Conway, 3rd Marquess of Hertford (1777-1842)
• Richard Seymour-Conway, 4th Marquess of Hertford (1800-1870)
• Francis Hugh George Seymour, 5th Marquess of Hertford (1812-1884)
• Hugh de Grey Seymour, 6th Marquess of Hertford (1843_1912)
• George Francis Alexander Seymour, 7th Marquess of Hertford (1871-1940)
• Hugh Edward Conway Seymour, 8th Marquess of Hertford (1930-1997)
• Henry Jocelyn Seymour, 9th Marquess of Hertford (b. 1958)

his son and heir: William Francis Seymour, Earl of Yarmouth (b. November 2, 1993) (until 1997: Viscount Beauchamp)