Joan arranged to marry John to Margaret, daughter of Edward III of England, but Henry of Grosmont, 1st Duke of Lancaster, an ally of the Montforts, broke off the negotiations.

In 1356, John was given to the English as a hostage and he was held at Gloucester Castle under the care of Robert de Vere, Duke of Ireland and Earl of Oxford for nearly twenty-five years.

On the death in 1380 of Charles V of France, who supported the Penthièvre claim against the Montforts, John and his mother negotiated the second treaty of Guérande on 14 April 1381, in which he renounced his claim to the Duchy of Brittany in exchange for a substantial indemnity.

John was released with financial assistance from Olivier de Clisson, who paid his 60,000 franc ransom and offered him his younger daughter, Marguerite, as a wife.

He shared with his three brothers the family inheritance: Olivier received Penthièvre, John the lands of Aigle in Normandy, Charles the lordship of Avaugour, and William the viscounty of Limoges.