As part of a diplomatic settlement in 1181, twenty-year-old Constance was forced into marriage with Geoffrey Plantagenet, the fourth son of Henry II of England and Eleanor of Aquitaine. With Geoffrey she had three children: Eleanor, Fair Maid of Brittany (1184–1241), Matilda/Maud of Brittany (1185 – bef 1189) and Arthur (1186–1203), the last born after Geoffrey's death. As de jure uxoris Duke, Geoffrey excluded Constance from exercising authority in government. In 1186, in a riding accident in Paris, Geoffrey was stamped to death during a tournament. Constance thereafter became the effective ruler of Brittany. Constance died, age 40, on 5 September 1201 at Nantes. She was buried at Villeneuve Abbey in Nantes.