He was also a friend and a comrade in arms of Bertrand du Guesclin, a fellow Breton Knight and Constable of France.

They had three daughters who married into the most prominent breton families of the time: During the War of the Succession in Brittany (1341–1365), Jean embraces the cause of Charles de Blois against John de Montfort for the ducal crown of Brittany and is one of the heroes who stands out most at the battle of La Roche-Derrien (1347).

Robert Bemborough, the English captain of Ploërmel, who supported the rival claimant John de Montfort, was the nearest enemy leader.

The battle, fought with swords, daggers, spears, and axes, mounted or on foot, was extremely vicious.

When his faction was eventually defeated at the Battle of Auray in 1364, de Beaumanoir helped to negotiate the Treaty of Guérande, which ended the war, receiving in return the title of Marshal of Brittany.