Edouard de Castelnau was born in Aveyron in Saint-Affrique. He was married in Toulouse on November 29, 1878 to Marie Francoise Jeanne Barthe of Mandegourg. Together they had twelve children.
He lost three of his sons during the World War 1.
On August 2, 1911, he was named first assistant manager of general staff under the orders of Joffre. He was promoted commander of the legion of honor the same year. At the end of 1913, he entered to the Higher Council of the War. He played a decisive role at the time of the battle of Verdun.
Edouard de Castelnau died in the castle of Lasserre with Montastruc the Adviser, buried in the new family vault in Montastruc.