Margaret was the elder of the two daughters of Francis I, Duke of Brittany (died 1450), by his second wife, Isabella of Scotland.

[1] Since the Breton War of Succession, Brittany had been understood to operate according to the semi-Salic Law: women could only inherit if the male line had died out.

As expected from the provision of the Treaty of Guérande, which ended the war, neither Margaret nor her younger sister Marie were recognized as heirs to the duchy.

He was also childless, and to avoid any subsequent dispute, he arranged the marriage of Margaret to his first cousin, Francis of Étampes, the second in the order of succession and the last male left of the Breton House of Montfort; also, he arranged the marriage of the younger sister Marie to John II, Viscount of Rohan, the most powerful noble in Brittany.

On 13 or 16 November 1455, Margaret was married to Francis of Étampes,[2] at the Château de l'Hermine in Vannes.