Charles was born on 6 October 1610, the second son of Léon de Sainte-Maure, baron de Montausier. His parents were Huguenots, and he was educated at the Protestant Academy of Sedan under Pierre Du Moulin. He served brilliantly at the siege of Casale in 1629. Becoming baron de Montausier at the death of his elder brother in 1635, he was the recognised aspirant for the hand of Julie d'Angennes, the marquis and marquise de Rambouillet's eldest daughter. Having served under Bernard of Saxe-Weimar in Germany in 1634, he returned to the French service in 1636, and fought in the Rhenish campaigns of the following years. He was taken prisoner on 25 November 1643 after the defeat of the French forces under the command of Josias von Rantzau in the Battle of Tuttlingen. He remained ten months in captivity until payment of his ransom was made.