He was made a Knight of Malta in 1606 at the age of 13 and became an officer of the galley. Thereafter he went to Paris where he earned a reputation as an impressive swordsman, though one regularly spoiling for a fight.
In 1626, he foiled an attempted assassination of Cardinal Richelieu.
After contributing to the defense of the island of Ré during the Siege of Saint-Martin-de-Ré, Valençay commanded the Royal Navy during the Siege of La Rochelle in 1628 with the rank of Vice-Admiral. He was promoted to Brigadier at age 34.
Her served under Duke Henri II de Montmorency at the Battle of Avigliana and was awarded the Bailiff Grand Cross of the Order of Malta for his service, during which he was wounded. However, he fell into disgrace when he took up arms with the Duke who sought to raise an army against Cardinal Richelieu. The Duke was captured at the Battle of Castelnaudary and executed; Valençay escaped to Malta.
In 1643 he was elevated to Cardinal (in pectore - it was published the following year) by Pope Urban VIII who hoped that making a large number of French-aligned cardinals would allow his Cardinal-Nephew, Antonio Barberini, to control the College of Cardinals and elect a pope friendly to the Kingdom of France. Pope Urban died the following year and Achille d'Étampes de Valençay participated in the papal conclave of 1644. For a number of reasons, Urban's plan did not work and Pope Innocent X (more friendly to Spain than France) was elected.
- "Achilles to Étampes-Valencay" in Marie-Nicolas Bouillet and Alexis CHASSANG (dir.), Dictionnaire universel d'histoire et de géographie, 1878
- Note: regardless of his previous conflict with Richelieu, it seems Valençay remained loyal to France.