[a] In December 1335, negotiations were made for a marriage between Joan and John of Eltham, Earl of Cornwall, the brother of King Edward III of England;[b] however, it appears these arrangements did not result in an official betrothal.

[4] In 1341, on the death of John III, the couple assumed the rule of the Duchy of Brittany, Charles having been granted permission to perform homage by King Philip VI of France by the arrêt of Conflans on 7 September 1341.

[e] The contest between the two claimants was then settled in 1365 by the First Treaty of Guérande; by its terms, Joan received a substantial pension (payments of which continued until 1372) in compensation for her claims, the right to maintain the ducal title for life, all her familial lands of Penthièvre and Avaugour, and an exemption from homage to the new duke for these territories.

In 1379, when John IV had been forced into exile in England, King Charles V of France attempted to annex Brittany to the French royal domain.

Both her supporters and those of the Montfort line united to invite John IV back from his exile in England and retake control of the duchy.

From the legal perspective of the Treaties of Guérande, the issue of succession to the ducal crown appeared settled, although Joan's descendants provoked various conflicts with John IV and future dukes from the House of Montfort.