She married the English prince John of Gaunt, who fought to obtain the throne of Castile in her name, but ultimately failed.

On 21 September, at Roquefort, near Bordeaux, Guienne, she married the English prince John of Gaunt, 1st Duke of Lancaster.

On 9 February 1372 Constance made a ceremonial entry into London as the queen of Castile, accompanied by John's eldest brother, Edward, the Black Prince, and an escort of English and Castilian retainers and London dignitaries.

Crowds lined the streets to see her as she processed to the Savoy Palace in the Strand where she was ceremonially received by her husband, who had proclaimed himself king of Castile on 29 January.

[3] John insisted that English nobles address him as "my lord of Spain", but was unsuccessful in his attempts to obtain the crown.