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
Edward Seymour, 8th Duke of Somerset and wife Elizabeth Somner, daughter of John Somner, of Seend. They had at least five children. In December 1740, his father died and Seymour inherited his manors in Wiltshire and Devon. On 11 September 1744, with the unexpected death of George Seymour, Viscount Beauchamp (1725–1744), the only son of Algernon Seymour, 7th Duke of Somerset, the likelihood emerged of Seymour succeeding his distant cousin as Duke of Somerset and Baron Seymour, as the 7th Duke was then aged sixty, and his wife was past child-bearing age. On 23 November 1750 the Duke died, and Seymour duly succeeded to his titles.