Catherine was the youngest daughter of the Spanish rulers Ferdinand II of Aragon and Isabella I of Castile. In 1501 she married Prince Arthur, eldest son of King Henry VII of England. Arthur died the following year, and shortly afterward she was betrothed to Prince Henry, the second son of Henry VII. But subsequent rivalry between England and Spain and Ferdinand's refusal to pay the full dowry prevented the marriage from taking place until her fiancé assumed the throne as Henry VIII in 1509. For some years the couple lived happily. Catherine matched the breadth of her husband's intellectual interests, and she was a competent regent while he was campaigning against the French (1512–14).

Between 1510 and 1518 Catherine gave birth to six children, including two sons, but all except Mary (later queen of England, 1553–58) either were stillborn or died in early infancy. Henry's desire for a legitimate male heir prompted him in 1527 to appeal to Rome for an annulment on the grounds that the marriage had violated the biblical prohibition against a union between a man and his brother's widow. Catherine appealed to Pope Clement VII, contending that her marriage to Henry was valid because the previous marriage to Arthur had never been consummated.

For seven years the Pope avoided issuing the annulment because he could not alienate Catherine's nephew, the Holy Roman emperor Charles V. Finally Henry separated from Catherine in July 1531. On May 23, 1533—five months after he married Anne Boleyn—he had his own archbishop of Canterbury, Thomas Cranmer, annul the marriage to Catherine. Parliament passed the Act of Supremacy repudiating all papal jurisdiction in England and making the king head of the English church. Although Catherine had always been loved by the English people, Henry forced her to spend her last years isolated from all public life.