Dereham is known for his sexual indiscretions with Queen Catherine Howard, the fifth wife of King Henry VIII of England before she became queen. Their affair began late in 1538 and lasted until Catherine was made lady-in-waiting to the King's fourth wife, Anne of Cleves. Subsequently, Dereham was appointed a secretary at Hampton Court, an appointment possibly engineered by the dowager Duchess of Norfolk, Agnes Tilney, to silence him about Catherine's previous indiscretions.
When their past relationship was brought to the attention of Archbishop Thomas Cranmer by a member of the Dowager Duchess's household, he reported them to the King in a letter. This provoked an investigation which resulted in the arrests of the dowager Duchess, her son William Howard, 1st Lord Howard of Effingham, Thomas Culpeper, Queen Catherine herself, and eventually Lady Rochford, one of Catherine's ladies-in-waiting. Under interrogation, Dereham admitted to a pre-marital relationship with Catherine, but claimed that they had never been intimate after Catherine's marriage to the King. Furthermore, he claimed that he had been supplanted in her affections by Culpeper.
Cranmer was faced with the rumours of a pre-contract of marriage between Dereham and Catherine. Such a pre-contract would effectively have been as binding as marriage itself, especially if the couple had sealed the agreement with sexual relations, and as a result Catherine's marriage to the King would have been unlawful. However, no evidence exists to support this allegation; incriminating documents are thought to have been burned by the dowager Duchess, as it is documented that she raided Dereham's coffers and destroyed letters.
A supposed love letter from Catherine to Culpeper had been discovered, sealing her fate and that of all those implicated.
Dereham died a traitor's death at Tyburn, being hanged, drawn and quartered. Culpeper also died at Tyburn, but as he had been favoured by the King before his affair with Catherine, his sentence was commuted to beheading. Catherine and Lady Rochford were beheaded at the Tower of London on 13 February 1542. The dowager Duchess was eventually released.
- The visitation of Norfolk in the year 1563 By William Harvey, England. College of arms, Norfolk & Norwich Archeological Society, p. 105
- Weir, Alison (1991). "The Six Wives of Henry VIII", p. 413. New York: Grove Press.
- National Archives: "Catherine Howard loses her head. This letter written by Catherine Howard, fifth wife of Henry VIII, was used as evidence of her adultery. It helped to seal her fate."