Thomas spent his childhood in Wulfhall, outside Savernake Forest, in Wiltshire. Historian David Starkey describes Thomas thus: 'tall, well-built and with a dashing beard and auburn hair, he was irresistible to women'. A prominent Tudor courtier, Sir Nicholas Throckmorton, 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'.
The Seymour family's power grew during Henry VIII's marriage to Anne Boleyn, to whom Jane became a lady in waiting. Anne failed to give King Henry a son, however, which gave the Seymour brothers an opportunity to push their sister Jane in the King's direction. In fact, Henry married Jane eleven days after Anne's execution in May 1536, and she gave birth to their son and only child in October of the following year.
It was the elder brother, Edward Seymour, who benefited most from his sister's marriage to the King. Historians have speculated whether the division between Edward and Thomas began at that time, as Thomas unsurprisingly began to resent his brother and the relationship between them began to dissolve. Although Thomas was named Lord High Admiral, he was consumed by jealousy of his brother's power and influence.
In 1543, John Nevill, 3rd Baron Latymer, died leaving a wealthy widow, formerly Catherine Parr. An attachment then developed between Catherine and Thomas. Unfortunately for Thomas, Henry VIII also became interested in Catherine and eventually married her, having been impressed with her dignity and intelligence. Jealous of Seymour's attentions to Catherine, the King sent Thomas away on a diplomatic mission to the Netherlands.
Henry VIII died in January 1547, leaving Catherine one of the wealthiest women in England. Thomas had been made Master-General of the Ordnance in 1544 and Lord Warden of the Cinque Ports in 1545. He returned to court a few months before Henry's death and saw his brother Edward become Lord Protector of England and, in effect, ruler of the realm as Regent for his nephew, Henry VIII's minor son and successor, the short-lived Edward VI. As part of a 'unfulfilled gifts clause' left unmentioned in Henry's will, Thomas was granted the title Baron Seymour of Sudeley. However, Thomas' fervent desire was to unseat and replace his brother as Lord Protector.
Though Thomas Seymour's name had been linked to Mary Howard, Duchess of Richmond, he was still unmarried at the time of the King's death. One view is that Thomas schemed to marry either Princess Mary or Princess Elizabeth, Henry VIII's daughters by his first two marriages, and there were rumours that he attempted to pursue a relationship with Elizabeth, still in her early teens. If he hoped for such a marriage as a route to power, he was unsuccessful, though his secret marriage to Catherine Parr, Elizabeth's guardian, in late April of 1547 was viewed by some as an attempt to become close to the young princess. Certainly, many regarded this marriage as having occurred too quickly after the King's death. Anne Stanhope, Somerset's proud wife, disliked Catherine and Thomas and began to turn many people in court against them. To demonstrate her hatred, Anne kept the Queen's jewels, which by right were Catherine's. Princess Elizabeth, Catherine Parr's ward, had gone to live with her stepmother in Chelsea after Henry VIII's death. Thomas, therefore, acquired the guardianship of Elizabeth and also of Lady Jane Grey, another young member of the household. The overly-ambitious Thomas started to make advances toward Elizabeth, sneaking into 'the Lady Elizabeth's chamber before she was ready, and sometimes before she did rise; and if she were up he would bid her good morrow and ask how she did, and strike her upon the back or on the buttocks familiarly....' As gossip began to spread, Kat Ashley, Elizabeth's governess, implored Seymour to quit his bedroom antics with the princess. Indignant, Thomas retorted, 'By God's precious soul, I mean no evil, and I will not leave it!' Strange episodes followed as Catherine began to join her husband in his bizarre behavior with Elizabeth, once even joining her husband in tickling the princess in bed and on another occasion holding Elizabeth still as her black dress was cut by Thomas's sword into 'a hundred pieces.' Although Elizabeth's governess at one time averred that the Queen had found Elizabeth in Seymour's arms (implying a sexual encounter or close to it), she later withdrew the story. Catherine did, nevertheless, try to save Elizabeth's reputation by sending her away to the house of Anthony Denny in Hertfordshire. However, when Catherine died in childbirth in August of 1548, Thomas renewed his attentions to the Princess.
Thomas also bribed a man called John Fowler, one of King Edward VI's closest servants, from whom he received information that the King frequently complained about the lack of pocket money he received. Thomas smuggled money to the King and began to voice open disapproval of his brother's administrative skills. As Lord High Admiral, he was able to control the English navy, and he openly asked people for support in case of a coup. As admiral, he also encouraged piracy, allowing pirates safe passage in exchange for shares of their booty. He was completely and thoroughly indiscreet in his bid for power.
Thomas seems also to have hoped to finance a coup by bribing the vice-treasurer of the Bristol Mint, Sir William Sharington. Sharington was responsible for debasing the coinage in Bristol and he had been fiddling the account books and keeping the majority of the profit. When Thomas learned of the scheme, he blackmailed Sharington.
On the night of the 16th of January, Thomas broke into the King's apartments at Hampton Court Palace. He entered the privy garden and awoke one of the King's pet spaniels. Alerted by the dog, the guards arrested Thomas, and he was sent to the Tower of London. On 18th January, the council sent agents to question everyone associated with Thomas, including Princess Elizabeth, who by now was suspected of a sexual relationship with him and even of being pregnant with his child, and possibly of being involved with him in a plot to seize the throne from her half-brother, Edward VI.
On 22nd February, the council officially accused him of thirty-three charges of treason. Somerset delayed signing the death warrant, so the council went to Edward VI for his signature. On 20th March, Seymour was executed at the Tower, dying 'dangerously, irksomely and horribly.' His daughter by Catherine Parr, Mary Seymour, was placed in the care of the Duchess of Suffolk, Catherine Brandon. Mary should have been left wealthy, but her mother, dying at her birth, had left her entire fortune to Thomas. When Thomas was executed, the crown confiscated everything he had, including Catherine's bequest. The child appears to have died around the age of two, when she disappears from the historical record. The title 'Baron of Sudeley' passed to Catherine Parr's brother, William.
It is falsely alleged that upon hearing of his death, Princess Elizabeth remarked, 'Today died a man with much wit and not much judgment.' It seems true, however, that after nearly being seduced by Thomas Seymour, Elizabeth learned to be considerably more wary in her interactions with men.