Cavendish was promoted to captain in the 1st Regiment of Foot Guards and lieutenant-colonel in the Army on 1 June 1756[4] and served as an aide-de-camp to the Duke of Cumberland in Germany in Summer 1757 during the early stages of the Seven Years' War.[2] Promoted to colonel on 7 May 1758 and appointed an aide-de-camp to the King on 9 May 1758,[5] he served under Charles Spencer, 3rd Duke of Marlborough during the raid on St Malo in June 1758 and then took part in the raid on Cherbourg in August 1758.[2] He commanded the rear-guard during the re-embarkation following the disastrous battle of Saint Cast in September 1758 and, having been taken prisoner, gallantly offered to remain in captivity on the basis that he was a Member of Parliament. He was nevertheless released by the Duke of Aiguillon in an exchange for a French officer of equal rank in October 1758.[6]