On his father’s death, his lordship took up the army as a profession, and in June 1777 was gazetted to a Cornetcy (commission or rank of a cornet which was the lowest rank of a commissioned cavalry officer) in the 7th Queen’s Dragoons; proceeded as extra A.D.C. to Sir Henry Clinton to the seat of war in America, and in October was present at the storming of Forts Clinton and Montgomery.
In November was appointed to a Lieut.-Colonelcy, and on 10 December was promoted to a Troop in the 17th Light Dragoons present at the engagement of Monmouth Court House.
In 1778 raised the British Legion, of which he was given command and provincial rank of Colonel.
On the 13th April 1779 was appointed Major of the 38th regiment of Foot, with which he served in the affairs of Springfield and Elizabeth Town Point.
On the 2nd February 1781 was appointed Captain and Lieut.-Colonel in the Coldstream Guards, and on the 6th October 1789 exchanged to the Lieut.-Colonelcy of the 29th Regiment of Foot; became Brevet Colonel on the18th November 1790, and on the 5th December 1792 was appointed Colonel of the Regiment.
In November 1793 was given a brigade in the Expedition, which under Lord Moira, was intended to help the French Royalists of La Vendee and La Loire.
On the 3rd October 1794 was promoted to Major-General; served in Flanders, and the following year in Holland and North Germany, when he commanded the whole of the Light Cavalry.
Returned to England in 1796; in 1797 commanded a brigade of Cavalry at Weymouth, and was on the 7th August 1797 transferred to the Colonelcy of the 2nd Life Guards.
Promoted to Lieut.-General on the 1st January 1801 ; in the autumn of 1805 was ordered to Russia as Ambassador.