Beatrice was born sometime between December 1248 and 1249, the only child of John I of Montfort, Count of Dreux and Jeanne, Dame de Chateaudun.

Thus, Beatrice succeeded her father as ruling countess of Montfort at the age of about one year old.

In 1251, Jeanne married her second husband, John II of Brienne, Grand Butler of France.

Beatrice was married to Robert IV, Count of Dreux, Braine and Montfort-l'Amaury in 1260, when she was about eleven years old.

[4] As was the custom for female rulers at this point in time, he became the co-ruler with Beatrice and Count of Montfort by right of his wife after their wedding.