Seymour was the son of Edward Seymour, Lord Beauchamp (who predeceased his own father) by his wife Honora Rogers. He was the grandson of Edward Seymour, 1st Earl of Hertford, by his wife Lady Katherine Grey, a sister of Lady Jane Grey, "The Nine Days Queen", which thus gave him a distant claim to the throne through Katherine's descent from Mary Tudor, younger sister of King Henry VIII. He was the great-grandson of Edward Seymour, 1st Duke of Somerset (c. 1500–1552), the uncle of King Edward VI and Lord Protector of England.
Seymour made a secret marriage at Greenwich on 22 June 1610 to Arbella Stuart (died 1615), daughter of Charles Stuart, 1st Earl of Lennox and Elizabeth Cavendish. Arbella was thirteen years his senior, and King James I disapproved of the marriage as the union of two potential Tudor pretenders to the throne, who were respectively fourth and sixth in line, could only be seen as a threat to the ruling dynasty. As a result, William was condemned to life imprisonment in the Tower of London and thus became the fourth of five generations of Seymours to spend time in that prison.
In June 1611 Seymour escaped from the Tower, planning to meet up with Arbella, who also had escaped captivity. They were to flee to the Continent, but bad weather and other circumstances prevented their meeting, and Arbella was recaptured and placed back in the Tower. William managed to reach safety abroad at Ostend, but was never reunited with Arbella who remained in the Tower until her death in 1615.
Seymour was knighted as a Knight of the Bath in November 1616. In December 1620 Seymour was elected Member of Parliament for Marlborough in Wiltshire, but vacated the seat soon afterwards on his elevation to the House of Lords, having succeeded his grandfather as Earl of Hertford in 1621. In the House of Lords he became a prominent opponent to King Charles I, where he supported the Petition of Right of 1628, and co-signed the letter of the twelve Peers of 1640, along with his brother-in-law Robert Devereux, 3rd Earl of Essex.
During the Long Parliament, Seymour eventually parted company with the more radical opponents of the King, and in 1641 was created by him Marquess of Hertford. In the Civil War, Hertford was a moderate royalist, along with such figures as Sir Edward Hyde, and throughout sought a compromise settlement, by continuing unofficial negotiations throughout the war with his brother-in-law Essex, the Parliamentary commander. He was nevertheless a trusted supporter of the King, who made him guardian of his son the future King Charles II, and he undertook several important military commands in royalist service over the course of the war, including commanding troops from South Wales. Along with his brother Lord Seymour, Hertford was sent as a commissioner for the king at the failed Treaty of Uxbridge negotiations (1645).
After the end of the First Civil War and the King's imprisonment, Hertford was the most prominent nobleman to remain alongside the king throughout his captivity, and was with him until his execution in 1649. He was one of four lords (the others being the Duke of Richmond, and the earls of Lindsey and Southampton) who petitioned the Commons to be allowed to assume responsibility for the King's actions and to suffer death in his place. During the Inter-regnum, Hertford kept himself away from both politics and royalist conspiracies, in the belief that the monarchy would eventually be restored and that conspiracies would only delay that event.
When the Restoration of the Monarchy came in 1660, Hertford was restored to all his former positions, and his services in the Royalist cause were further recognised by King Charles II, who in 1660 restored Hertford to his great-grandfather's Dukedom of Somerset, which had been forfeited in 1552. He thus became the 2nd Duke of Somerset.
Marriages & children
William Seymour married twice:
Firstly and secretly (see above), on 22 June 1610, to his cousin Lady Arbella Stuart (died 1615), who was then fourth in line to the succession of their cousin, King James I. There were no children from the marriage.
Secondly, on 3 March 1617 at Drayton Bassett, he married Lady Frances Devereux (1599–1674), daughter of Robert Devereux, 2nd Earl of Essex, by his wife Frances Walsingham, daughter of Francis Walsingham. By Frances he had at least eight children:
- William Seymour (1621 – 16 June 1642), who predeceased his father.
- Robert Seymour (1622–1646), who predeceased his father.
- Henry Seymour, Lord Beauchamp (1626 – 30 March 1654), who married Mary Capell and had children, including William Seymour, 3rd Duke of Somerset (1654–1671), heir to his grandfather the 2nd Duke.
- Edward Seymour, who died in infancy.
- John Seymour, 4th Duke of Somerset (1646 – 29 April 1675), heir to his nephew in the dukedom in 1671, who married Sarah Alston in 1656 but left no children.
- Lady Frances Seymour (1618–1685), who married three times: Firstly to Richard Molyneux, 2nd Viscount Molyneux Secondly (as his third wife) to Thomas Wriothesley, 4th Earl of Southampton, and thirdly (as his third wife) to Conyers Darcy, 2nd Earl of Holderness, by whom she had children.
- Lady Judith Seymour (1623–1645), who married Peter Ransone, by whom she had issue.
- Lady Mary Seymour (1637 – 10 April 1673), who married Heneage Finch, 3rd Earl of Winchilsea and had issue.
- Lady Jane Seymour (1637 – 23 November 1679), who married Charles Boyle, 3rd Viscount Dungarvan and had issue. Her modern descendants included Elizabeth Bowes-Lyon.
Death, burial & succession
- "The story of Lady Arbella Stuart". National Trust. Retrieved 10 June 2019.
- Norman Egbert McClure, Letters of John Chamberlain, vol. 2 (Philadelphia, 1939), p. 31.
- One or more of the preceding sentences incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). "Somerset, Earls and Dukes of". Encyclopædia Britannica. 25 (11th ed.). Cambridge University Press. p. 385.
- Rushworth, John (1721) [orig. before 1690]. "Historical Collections: The treaty at Uxbridge, 1645". In Browne, D (ed.). Historical Collections of Private Passages of State: Volume 5, 1642-45. London. pp. 787–843. Retrieved 24 May 2020 – via British History Online.
- "Seymour, William, 1st marquess of Hertford, 2nd duke of Somerset," Oxford Dictionary of National Biography. The first edition of this text is available at Wikisource: . Dictionary of National Biography. London: Smith, Elder & Co. 1885–1900.