He was son of John de Montfort and Joanna of Flanders. His father claimed the title John IV, Duke of Brittany
He married three times:
1) Mary Plantagenet (1344–1362), daughter of King Edward III of England and Philippa of Hainault.
2) Joan Holland (1350–1384), daughter of Thomas Holland, 1st Earl of Kent, in London in May 1366.
3) Joanna of Navarre (1370–1437), daughter of King Charles II of Navarre, at Saillé-près-Guérande on October 2, 1386.
Joanna was the mother of all his children. After his death she married Henry IV of England.