On the death of his father on 30 Mar. 1654 Seymour became heir to his grandfather William Seymour, marquess of Hertford, and ultimately to the dukedom of Somerset. Lord Beauchamp, his sister, Lady Elizabeth Seymour, and their mother all lived with Hertford until his mother remarried in August 1657. Her new husband was Henry Somerset, styled Lord Herbert, the future 3rd marquess of Worcester and duke of Beaufort, whose father, Edward Somerset, 2nd marquess of Worcester, was the rival claimant to the Somerset dukedom. Almost immediately Hertford made a new will in an attempt to ensure that his grandson and the Seymour patrimony would remain as far as possible in the hands of his wife, the marchioness of Hertford, supported by trustees who included her sons-in-law Thomas Wriothesely, 4th earl of Southampton, and Heneage Finch, 3rd earl of Winchilsea.
In September 1660 Hertford secured a private act of Parliament establishing his claim as duke of Somerset. He therefore died at the end of October as 2nd duke of Somerset, and William Seymour succeeded as 3rd duke. At the death of the old duke, the new duke’s mother appealed to the king in an attempt to know why his grandmother retained custody of her son, or ‘permit her to recover his guardianship by law’. In response, it was explained that the duke had been ‘willing for his grandson and heir to remain under the tuition of his mother, only so long as she remained a widow’ and that, upon her marriage with Lord Herbert, he had obtained a promise from the king to grant him, as far as he could, the wardship of his heir. He had bequeathed a large property to the marchioness, on condition of her retaining the wardship of the heir. The whole estate was burdened with debt, but the marchioness was willing to allow the proceeds to go to discharging the debts, and to educate her grandson at her own expense.
The scene now shifted to Parliament and the bill to abolish the court of wards. In December the dowager duchess petitioned for a clause to allow the king to grant the wardship in this particular case; Lady Herbert counter-petitioned.
In an attempt to sort out the tangled finances of the family, one of the old duke’s trustees, Sir Orlando Bridgeman‡, drafted a bill which was introduced into the Lords on 24 Mar. 1662 as a bill for making provision for the speedy payment of the debts of the late duke of Somerset. It received a second reading on the following day, but opposition from the dowager duchess helped to ensure that it was never reported from committee. Its main purpose seems to have been to ‘join the whole estate of the old duke together and enable trustees to pay the debts out of the entailed estate as well as the trust’, as the only way ‘to preserve the family’.
Somerset spent his teenage years with his mother at Badminton and Worcester House in London. However, the young duke appears to have been impatient to secure his independence, declaring that he would ‘remove to Tottenham the day after he should come of age’.
In July 1668 a marriage was under discussion between Somerset and the sister of James Scott, duke of Monmouth, but these plans never came to fruition.
Somerset had been ‘let go out of his mother’s constant care and inspection to come up to court the Countess of Northumberland’, which allegedly brought him into the company of a group of ‘young men’ who apparently introduced him to ‘liberties before unknown to him’.
Somerset died at Worcester House, London, on 12 Dec. 1671, ‘a person every way so healthy, vigorous and young’ but nonetheless ‘hurried away in five days’ time’.
Somerset’s body was conveyed from London via Reading to Great Bedwyn, where he was buried on 20 December.
