Sir Thomas Seymour was the fourth son of his parents, born some time before 1509. Nothing is known of his education, or childhood, his first appearance in records dating from the early 1530s when he was in the train of his cousin, Sir Francis Bryan, English Ambassador to France. Seymour's role was as messenger between the Ambassador and Henry VIII.
Sir Thomas' real break came when his sister, Jane, caught the King's eye and married him in May 1536. As the brother of the Queen, he was rapidly advanced to a place in the Privy Chamber. The most important development in his career was his grant of a naval command which enabled him to take part in victorious skirmish with French ships. In October 1537, he was knighted and took part in the christening of his nephew, Edward, Prince of Wales.
After Queen Jane's death, he remained on good terms with his brother-in-law, to the extent that he felt able to criticise Archbishop Cranmer, one of the very few men Henry never turned against. He accused Cranmer of misappropriating funds, but had the grace to withdraw the accusation when proven wrong.
Around this time, the first marriage plans for Seymour were mooted. Such a late time in life for a first marriage plan seems very unusual, but there is no record of any earlier marriage. The suggested bride was the King's own daughter-in-law, Lady Mary Howard, Dowager Duchess of Richmond. It seems her father, the 3 rd Duke of Norfolk, favoured the match but that the Duchess herself, and her brother the Earl of Surrey, were very much against it. Seymour's feelings are unknown – although it seems likely he would have been keen on the match with a daughter of the premier ducal house, in close relation to the King.
Seymour continued to perform various duties around the court – embassies to France and the Low Countries and a position in the group of dignitaries sent to greet Anne of Cleves at Calais. All of this suggests that he was a man of some charm and diplomacy. He was also a skilled tilter and took part in the tournaments arranged to welcome Anne of Cleves.
At some time in early June 1542, he seems to have begun courting Katherine Parr, the widowed Lady Latimer. She had been left comfortably off by her late husband, but by no means rich, so the two must have been drawn together by genuine attraction. Unfortunately for Sir Thomas, he had a rival whom no amount of good-looks, charm or courage could outshine - the King himself.
Sir Thomas was dispatched by Henry in June 1542 as ambassador to Ferdinand, King of Hungary, brother of the Emperor Charles. He took part in various battles to defend Christendom against the encroaching Turks, before returning to England in 1543. He was again sent on embassy, this time to the Low Countries, where he was second-in-command in the English contingent, supporting the Emperor in his never-ending wars against the French. He criticised the Emperor freely, obviously to Henry VIII's satisfaction as he was given the role of Master of the Ordnance for life.
In Henry VIII's will, Seymour was named as an assistant to his executors, The Council rode rough-shod over the details of the will and Seymour was appointed to the Privy Council. This was presumably to please his brother, Edward Seymour, now Duke of Somerset, who arrogated to himself supreme power as Lord Protector. Sir Thomas, until now apparently on good terms with his elder brother, fell into the trap of a vicious sibling rivalry, not helped by his covert marriage to the Queen Dowager, his old flame Katherine Parr. The brothers and their respective wives fell out over the spoils of the late King's will, and both tried to manipulate the Council, and the new king. The elder brother, Edward, emerged victorious, but Thomas continued to sulk and create difficulties, despite receiving the title of Baron Seymour, and being granted the delightful Sudeley Castle.
Seymour also began to indulge in extremely inappropriate behaviour with his wife's step-daughter, the fourteen-year-old Elizabeth, later Elizabeth I. He would enter her bedroom before she was dressed, tickle her in bed, and ask after her “great buttocks". Eventually, the threat to the young princess' reputation became so great that Queen Katherine sent her away. Seymour and his wife were reconciled, and had the happy expectation of a child to draw them together. Throughout 1548 Seymour and his brother continued to quarrel, despite there being enough family feeling left for Somerset to congratulate him on the birth of a daughter in September of that year.
Sadly, Sir Thomas gained a daughter but lost a wife. He mourned for a short period, but soon took up the idea of marrying the Lady Elizabeth. Such a scheme was crazy at best – to marry the King's sister would take him too close to the throne, and could never be approved by the Lord Protector and the Council. Nevertheless, he persisted with the scheme, and also seems to have embroiled himself in piracy and counterfeiting.
On 17 th January 1549 he was arrested and charged with 33 counts of treason, many of which related to his conduct with Lady Elizabeth. He was not tried by his peers, as he was entitled to be. Instead, an Act of Attainder was pushed through Parliament, being passed on 5 th March 1549, enabling a sentence of treason to be passed without trial, and execution as the result. He lost his head on 20 th March, his last request being that his daughter be placed in the care of her mother's friend, Katherine Willoughby, Dowager Duchess of Suffolk.
Seymour was described by Sir Nicholas Throckmorton, a cousin of his wife as “hardy, wise and liberal ... fierce in courage, courtly in fashion, in personage stately, in voice magnificent, but somewhat empty of matter." The most telling epitaph, perhaps, was that of the fifteen year old Lady Elizabeth, who, on being informed of his death apparently said “This day died a man of much wit, but very little judgement".