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, 1st Duke of Somerset to throw off the Protector within two years and "bear rule as other kings do"; but Edward, schooled to defer to the Council, failed to co-operate. In April 1547, using Edward's support to circumvent Somerset's opposition, Thomas Seymour secretly married Henry VIII's widow Catherine Parr, whose Protestant household included the 11-year-old Lady Jane Grey and the 13-year-old Princess Elizabeth. In summer 1548, a pregnant Catherine Parr discovered Thomas Seymour embracing Princess Elizabeth. As a result, Elizabeth was removed from Catherine Parr's household and transferred to Sir Anthony Denny's. That September, Catherine Parr died in childbirth, and Thomas Seymour promptly resumed