He distinguished himself during the Thirty Years' War as lieutenant general of the King's Armies in Flanders and was elevated to the dignity of Marshal of France 3 weeks before his death.

He fought against the Spanish in the and distinguished himself in 1636 during the Sieges of Corbie and La Capelle, where he was taken prisoner and imprisoned in the citadel of Cambrai from where he escaped.

Wounded in 1638 and again in 1639, he distinguished himself during the Siege of Arras (1640), where he acquired a reputation as a brave and experienced soldier.

In 1658, he commanded the left wing of the French Army, assisted by an English contingent, during the Battle of the Dunes on 14 June.

de Castelnau is remembered with a buste in the Galerie des Batailles in the Palace of Versailles.