During his marriage to Catherine, Seymour involved the future Queen Elizabeth I (then 14 years old), who resided in his household, in flirtatious and possibly sexual behaviour.

Henry married Jane eleven days after Anne's execution in May 1536, and the Seymour brothers saw their fortunes rise: in that year, Thomas became a gentleman of the privy chamber.

[7] A few weeks later he was sent to King Ferdinand I of Hungary, brother of Emperor Charles V, to enlist support for Henry against France and Scotland.

[9] When war broke out between England and France, Seymour was made marshal of the English army in the Netherlands on 26 June 1543, being second in command to Sir John Wallop.

[12] Seymour returned to court just before Henry VIII died in January 1547, leaving Catherine one of the wealthiest women in England.

[14] Thomas Seymour sought to overturn his brother's position on the regency council by his personal influence over the young king, and also possibly by making a royal marriage.

[6] Anne Stanhope, Somerset's proud wife, disliked Catherine and Thomas and began to turn many people in court against them.

[17] Elizabeth's feelings regarding this behaviour are unknown, but it was said that she bore Thomas some degree of affection;[18] and though her governess "bade him go away in shame," she found him more amusing than dangerous.

Given Elizabeth's relatively young age, the fact she was Catherine's step-daughter and also second in the line of succession of the English throne plus Seymour was married to the dowager queen, his behaviour was considered to be highly inappropriate, shocking, and immoral.

[20] Despite his great wealth and high position, Thomas Seymour could not come to terms with his brother's appointment as protector; and in his struggle with Somerset, he tried to ingratiate himself with the king, who was merely a child.

Even though he lived in sumptuous splendour and wanting for nothing, no provisions had been made for Edward's pocket money; he became accustomed to these regular payments and began to ask Seymour freely for his allowance.

Seymour intended that the king's royal signature and personal support would destabilize Somerset's position as protector, and as a member of the regency council.

In his frustration and inability to gain any significant influence over the king, Thomas Seymour began to think in terms of open rebellion.

[22] Thomas seems also to have hoped to finance a rebellion through crooked dealings with the vice-treasurer of the Bristol Mint, Sir William Sharington.

The incident, being caught outside the king's bedroom, at night, with a loaded pistol, was interpreted in the most menacing light, even casting suspicion on Elizabeth's involvement with Thomas.

[25] Seymour was a "trusty friend" of Sir Rowland Hill, who would publish the Geneva Bible, and was given land at Hoxten for life under his will.

[28] But the council found itself in a sharply defined game of wits with Elizabeth, who proved to be a master of logic, defiance, and shrewdness.

His attainder was reversed by Parliament in 1550, although the property was not returned to Mary Seymour, his only child; she is believed to have died at about the age of two, possibly while in the care of Katherine Brandon, Duchess of Suffolk.

Sir Nicholas Throckmorton, a boyhood friend of King Edward, described Thomas Seymour as "hardy, wise and liberal ... fierce in courage, courtly in fashion, in personage stately, in voice magnificent, but somewhat empty of matter".