John of Brittany served England as a soldier and as a diplomat but was otherwise politically inactive in comparison to other earls of his time. He was a capable diplomat, valued by both Edward I and Edward II for his negotiating skills. John was never married, and upon his death his title and estates fell to his nephew, John III, Duke of Brittany. Although he was generally loyal to his first cousin Edward II during the times of baronial rebellion, he eventually supported the coup of Isabella and Mortimer. After Edward II abdicated in favour of his son Edward III of England, John retired to his estates in France and died in his native Brittany in 1334 with no known issue.