Uncertainty exists around the birth of du Casse. Though he is usually said to have been born on 2 August 1646 in Saubusse, near Dax (Landes), the son of Bertrand Ducasse, a Bayonne ham merchant, and Marguerite de Lavigne, he was actually born in Pau to Jacques Ducasse and Judith Remy. His father Jacques was the son of Gaillard Ducasse, a minister in the Reformed Church of France, which designated the family as Huguenots. Because Huguenots were persecuted at this time in France, and their career prospects were limited, it is thought that du Casse forged his baptismal record to hide his Huguenot and ignoble background. Early biographers, including his grand-nephew and Saint-Simon, perpetuated this error in their works.