Through his paternal grandmother, he was a great-great-grandson of Mary Tudor, Queen of France and Charles Brandon, 1st Duke of Suffolk, thereby making him a direct descendant of Henry VII.

In June 1611 he was accused of abetting the escape of his brother William Seymour and his wife Arabella Stuart, but protested his innocence.

In that parliament he worked hard to induce a war with Spain, but protested against any extensive military operations in continental Europe and opposed sending an army to the Palatinate on the ground of the "extreme charge".

On 30 July he proposed that the grant be limited to one subsidy and one-fifteenth, about a tenth of what King Charles I required to meet his engagements.

In August he attacked the government for conducting a continental war, inveighing against peculation in high places and the sale of offices at court.

[3] In May 1639 Seymour refused to pay ship-money, and in the following March he was elected without opposition as MP for Wiltshire to the Short Parliament.

In this parliament he spoke powerfully against granting any subsidies to the King before receiving any redress of grievances, and apparently compared "our affayres to the bondage of the Israelites in Egypt".

In December 1643 he signed the letter of the peers to the council in Scotland, protesting against the invitation sent by parliament to the Scots to invade England.