Marriage to Prince John at Marlborough

On 29 August 1189, Isabella of Gloucester married Prince John, the youngest son of King Henry II and the future King John of England, at Marlborough. The marriage had been arranged years earlier, in 1176, as part of Henry II's strategy to endow his landless youngest son with wealth and title through the rich inheritance of the earldom of Gloucester, of which Isabella was a co-heiress following the death of her father William, 2nd Earl of Gloucester, and the exclusion of her elder sisters from the succession to the earldom.

The union was politically significant rather than romantic: it transformed John from a prince with few prospects into one of the wealthiest magnates in England, holding the extensive Gloucester estates in right of his wife. The marriage was controversial from the outset because Isabella and John were second cousins, sharing a great-grandparent in Henry I, placing them within the prohibited degrees of consanguinity under canon law. Papal dispensation was not obtained, which would later provide the legal grounds for the marriage's annulment.

This marriage set the stage for Isabella's unusual status as a woman who married a future king yet never became a crowned queen, an unprecedented situation for an English royal consort since the Norman Conquest. It illustrates the medieval practice of using aristocratic marriage as a tool of political and economic consolidation.