He was styled Viscount Petersham from 1756 until 1779 when he succeeded his father as 3rd Earl of Harrington. Educated at Eton from 1767 until 1769, he entered the Army as Ensign Coldstream Guards on 3 November 1769 to become Lieutenant Colonel of the 3rd Foot Guards and A.D.C. to General Burgoyne in the American War in 1777. 
From 1779 until 1783 he was Lieutenant-Colonel Commandant of 85th Foot; A.D.C. to the king in 1782 and 1783; Colonel of the 65th Foot 1783-1788; Colonel of the 29th Foot 1788-1792; Colonel of the 1st Life Guards and Gold Stick from 1792 until he died. In 1798 he became a Lieutenant-General and in 1803 a General.
In September 1774 he was M.P. for Thetford and for Westminster from 1776 until 1779. He was appointed Commander-in-Chief of the forces in Ireland 1805-1812. Also in 1805 he went as Ambassador on a special mission to Berlin and Vienna, then to Berlin only again in 1806. From 1812 until his death he was Constable of Windsor Castle.
On 22 May 1779 he married Jane Fleming, who had an independent fortune of œ100,000, and they became the parents of ten children. The purity of his domestic life was praised and contrasted with the immorality of his parents; and as a soldier, though "bordering on the martinet", he is stated to have been "beloved and respected both by officers and men".
Lady Harrington died in 1824 aged sixty-nine while the Earl of Harrington died in 1829 aged seventy-six.
 
 

Source: Leo van de Pas