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.

[2] 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.

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.