Seymour’s marriage into the Killigrew family brought him distant kinship ties to Sir Robert Cecil†, who accorded him ‘honourable respects’ in 1601, and presumably influenced his knighthood two years later. He sat for Penryn in 1601 and Newport in 1604 as a Killigrew nominee, but left no mark on the Commons’ proceedings, although he doubtless followed the progress of the estate bills introduced in these parliaments by his father. Returned for Lyme Regis in 1614, presumably by arrangement with John Drake*, he again failed to secure committee nominations or contribute to debate.
Seymour represented his county in the 1621 Parliament, but he still failed to make much impression on the Commons, and was nominated only to the committee for the bill against usury (7 May). In his first recorded speech, he called for Thomas Sheppard* to be committed to the serjeant in punishment for his diatribe against the Sabbath bill (16 February). Ten days later he briefly spoke up for his constituents, denouncing the local grievance of tithes imposed on fishermen which, he alleged, had caused ‘a great decay of mariners’. He also brought in an apologetic petition from the notorious patentee, Sir Giles Mompesson*, and urged the House to send for Sir George Marshall*, who had blatantly procured a knighthood of the Bath in return for money (24 Feb., 1 May). Seymour spoke only once during the winter sitting. After the king refused to receive the Commons’ petition supporting war with Spain, disagreement arose over how to present him with a replacement declaration. Seymour agreed that the Speaker should deliver it, ‘that it may appear to His Majesty how far we are stricken’.
In December 1622 Seymour was dismissed as vice admiral by the duke of Buckingham, following complaints to the Privy Council by French merchants, who claimed that he had embezzled stolen property brought into Plymouth by privateers. Despite this public disgrace, he bore no obvious ill will towards his successor, Sir John Eliot*, who presented him with two gifts of confiscated merchandise 18 months later.
In the 1624 parliamentary elections, Seymour was presented with a seat at Callington by his wife’s kinsman, John Trelawny. He clearly took an interest in the bill to break the entail on the estates of his cousin, the 2nd earl of Hertford, but left it to his friend Eliot on 10 Mar. to raise objections on his behalf. Seymour had evidently made his peace with Buckingham by now, and several times intervened in the Commons in support of his schemes. On 1 Mar. he objected to the king’s message banning further attacks on lord keeper Williams, the duke’s enemy, ‘saying that it trenched into the liberty of the House to have complaints stopped against any court of justice’. Later that day, ostensibly making the reasonable point that merchant ships should not be sent overseas with a war looming, as the Navy was thinly stretched, he called for five East India Company vessels to be stayed in London. However, Buckingham almost simultaneously made the same demand in the Lords, and it was widely believed that these interventions were really part of the duke’s bid to extort £10,000 from the Company. On 6 Mar. Seymour sought to increase the pressure on the East India merchants, moving that the ships should be searched and their account books examined, as they were said to have 40 chests of silver on board. Nevertheless, he was probably genuinely concerned about the impact on the money supply if bullion was exported, as on 12 Mar. he also reported that a similar danger was posed by Catholics who were stockpiling gold in London. The House accepted his suggestion that a message be sent to the Lords requesting that action be taken, and Seymour himself was the first Member named to the drafting committee. Seymour received only one other nomination, to consider the bill for restitution in blood of Carew Ralegh† (8 Apr.), and did not attend this committee.
In 1625 Seymour became the first member of his family to sit for Totnes, the borough closest to Berry Pomeroy, but he is not known to have contributed to the Parliament’s proceedings. He appears not to have sought election thereafter, though he featured in the records of the 1626 session, being cited in the impeachment charges against Buckingham as an accomplice in the duke’s manipulation of the East India Company.
The 1630s found Seymour less in sympathy with Crown policies. In 1635 he was summoned before the Privy Council for supporting a Devon petition against Ship Money. Three years later, he joined in protests against government interference in West Country trade. His appointment in 1638 as a gentleman of the Privy Chamber was probably intended to encourage greater co-operation, but in the following year he refused to contribute towards the costs of the First Bishops’ War.
Seymour died in October 1659, having passed his final years in retirement, and was buried at Berry Pomeroy. No will or grant of administration has been found. His son Edward sat for Totnes during most of Charles II’s reign, and his grandson, another Edward, served with distinction as Speaker of the Commons from 1673 to 1678.