Soldier and diplomat. Born at Petersham (England), Cathcart was educated at Eton and then the University of Glasgow, where he read law. He joined the army and served in America and Flanders. He was Commander-in-Chief in Ireland between 1803 and 1805, led the Siege of Copenhagen in 1807, capturing the Danish fleet, and thereafter briefly commanded the army in Scotland. He was promoted to a General in 1812 and soon after was sent as Ambassador to Russia, assisting Tzar Alexander I and the Russian Army against the French. During this time his son Sir George Cathcart (1794 - 1854) acted as his aide-de-camp. He was rewarded with a peerage (Baron Greenock and Viscount Cathcart) in 1807 and created the Earl of Cathcart in 1814.
He died at his home at Cartside, near Johnstone (Renfrewshire), with his son Charles Cathcart (1783 - 1859) succeeding him as the 2nd Earl.