England and Portugal entered into an alliance against Castile in 1386 and solidified their ties through the marriage of King John I and Catherine's half-sister, Philippa.

[2] John of Gaunt had ruled Santiago de Compostela, Vigo, and Pontevedra with ease, but had to withdraw to Portugal in 1387 because of an unsuccessful invasion of León.

It was then that he accepted the proposal of King John I of Castile, to marry Catherine to his son, the future Henry III, and that Constance, Duchess of Lancaster, should renounce all claims to the Castilian throne.

Furthermore, together with the Truce of Leulingham and the one made at Monção Municipality, the betrothal helped to end the Spanish period of the Hundred Years War.

The couple's three children: Henry III died in 1406, and according to his will, his widow, Catherine, and his brother, Ferdinand I of Aragon were to be joint regents during John II's minority, sharing their power with a royal council.

Catherine prepared to defend herself and her household in a famous Spanish castle, the Alcázar of Segovia, because she was not willing to relinquish her year-old son.

As Catherine became increasingly involved in the wars of Ferdinand against Granada in the south, Castile's alliance with France suffered and she was able to strengthen her relations with Portugal, where her half-sister Philippa was queen, and with England, where her half-brother Henry IV ruled since 1399.

Because of Catherine's opposition to Ferdinand, she supported the position of Antipope Benedict XIII and initially spoke up against the Council of Constance (1414–1418).

Queen Catherine died at Valladolid on 2 June 1418, of a stroke, leaving her thirteen-year-old son at the mercy of self-interested courtiers.