William Seymour, 2nd Duke of Somerset 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 (d.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
Duke of Somerset him Marquess of Hertford in 1640 and on 13 September 1660, shortly before his death on 24 October, the title of Duke of Somerset was granted to him as its legitimate heir; following its non-existence for 108 years. He outlived his three eldest sons and as the dukedom descends to heirs male of the holder of the 1547 grant it passed to William Seymour (1654–1671) who was the son of the third son mentioned (lived 1626–1654). The 3rd duke died unmarried and the title passed to John Seymour (bef. 1646–1675) the last surviving son of the 2nd Duke, his uncle.