Antoine de Castelnau was the son of Louis de Castelnau, baron of Castelnau, Miremont, Buanes and Bats,(1460–before 1529), and Susanne de Gramont (died after 1525).
The eldest branch of this family held the marquisate of La Ferté, and produced a heroine of the religious wars of the 16th century, Madeleine de St Nectaire, who married Guy de St Exupery, seigneur de Miremont, in 1548, and fought successfully at the head of the Protestants in her territory against the troops of the League.