According to Goff, Katherine likely spent her early childhood at Parham, as her mother was in almost constant attendance on Henry VIII's Queen,
Catherine of Aragon. On 14 October 1526, when Katherine was seven years of age, Lord Willoughby died after falling ill during a visit to Suffolk and was buried at
Mettingham. As his only surviving child, Katherine inherited the barony. Her father held some thirty manors in
Lincolnshire, and almost the same number in
Norfolk and
Suffolk, worth over £900 per annum, and Katherine is said to have been 'one of the greatest heiresses of her generation'. However, her inheritance became a subject of dispute for many years, as there was doubt as to which lands had been settled on the heirs male and which on the heirs general, and the matter was further complicated by a deed which Lord Willoughby had drawn up before leaving for France to campaign in Henry VIII's wars in 1523. In 1527 Katherine's uncle,
Sir Christopher Willoughby, accused his sister-in-law, Katherine's mother, María de Salinas, of withholding documents from him which established the title to various estates, and of having kept him out of possession of estates which rightfully belonged to him. At her father's death, Katherine's
wardship fell to the king, who on 1 March 1528 sold it to his brother-in-law
Charles Brandon, 1st Duke of Suffolk. On acquiring Katherine's wardship, Suffolk immediately intervened in the family quarrel with a letter to
Cardinal Wolsey, and his intervention appears to have cowed Sir Christopher Willoughby, who wrote to Wolsey that the Cardinal's anger was 'worse to him than death'. Katherine is said to have been betrothed to
Henry Brandon, 1st Earl of Lincoln (died 1534), Suffolk's son by his third wife,
Mary Tudor. Mary Tudor died at
Westhorpe, Suffolk, on 25 June 1533, and on 21 July the young Katherine was one of the chief mourners at her funeral. As early as 1531 it had been rumoured in the household of Henry VIII's future wife,
Anne Boleyn, that Suffolk was personally interested in Katherine, and six weeks after Mary Tudor's death the
Imperial Ambassador,
Eustace Chapuys, reported to
Charles V that: On Sunday next the Duke of Suffolk will be married to the daughter of a Spanish lady named Lady Willoughby. She was promised to the Duke's son, but he is only ten years old, & although it is not worth writing to your Majesty, the novelty of the case made me mention it'. Although Suffolk was forty-nine and Katherine only fourteen, the marriage was a financially successful one. The Willoughby inheritance was not fully settled until the reign of Queen
Elizabeth I, but Suffolk was able to force Sir Christopher Willoughby to relinquish possession of some of the contested Willoughby estates, and Suffolk eventually became the greatest magnate in Lincolnshire. As such, he played an important role in quelling the
Lincolnshire rebellion in 1536, and built an imposing residence at
Grimsthorpe, which came into Katherine's possession at the death of Elizabeth de Vere, Dowager Countess of Oxford, widow of the
13th Earl. The Duke and Duchess had two sons,
Henry Brandon, 2nd Duke of Suffolk, born 18 September 1534 at Katherine's mother's house in the Barbican, and
Charles Brandon, 3rd Duke of Suffolk, born 1537. The marriage brought Katherine into the extended royal family, because
Henry VIII's will made his younger sister Mary Tudor's descendants the next heirs to the throne after his own children. The Duke and Duchess of Suffolk officially greeted
Anne of Cleves when she arrived in England in 1539 to marry the king, and in 1541 they helped arrange a royal progress for the king and his next wife,
Catherine Howard. This progress later became notorious for the queen's alleged adulterous trysts with her kinsman,
Thomas Culpeper, though the duke and duchess's home at
Grimsthorpe Castle was "one of the very few places on the route ... where Catherine Howard had not misbehaved herself". ==Personality and beliefs==