Alix was the daughter of Constance, Duchess of Brittany and her third husband, Guy of Thouars. Alix was born on or shortly before the 5th of September 1201.

She was the twin sister of Katherine, Dame de Vitrem, and half sister to Arthur and Eleanor of Brittany, pawns in the political upheaval during King John's reign, held at Corfe Castle until their deaths.

The Breton barons recognized Alix as Duchess of Brittany after the death of Arthur, instead of her older sister, Eleanor, still captive at Corfe. Her father became regent for Alix until 1206, when King Philip II of France made himself the regent of the duchy in Alix's name.

In 1213, King Phillip arranged her marriage to his cousin Peter of Dreux, who became the regent. Alix and Peter had three children:
John, Duke of Brittany
Yolande, wife of Hugh XI of Lusignan
Arthur, died at three years

Alix died in childbirth, succeeded in the duchy by her son John I, but Peter remained the de facto ruler of Brittany until 1237.