Soldier and diplomat. Born in Edinburgh, the son of another Charles Cathcart (8th Lord Cathcart) and Marion Shaw. He fought at Dettingen in 1743 and served as an aide-de-camp to William Augustus, Duke of Cumberland, at the Battle of Fontenoy (1745), where he was shot in the face. He wore a black silk patch to cover the scar in his portrait by Joshua Reynolds and this earned him the soubriquet 'Patch Cathcart'. He served Cumberland at the Battle of Culloden in 1746 and was again wounded. He subsequently held a number of senior military posts in Scotland; was a High Commissioner to the General Assembly of the Church of Scotland, and was Scotland's first Lord Commissioner of Police.
He was a Representative Peer (1752-76), was promoted to Major-General (1758), Lieutenant-General (1760) and held the office of Governor of Dumbarton Castle (1761-64). He was appointed Ambassador to Russia (1768-72), becoming a friend of Catherine the Great, and served as Rector of the University of Glasgow (1773-75).
He inherited his family estate of Sundrum (South Ayrshire) but sold these to James Murray of Broughton in 1758 because he has also fallen heir to his mother's estates in Greenock.
Cathcart died in London. His son William was create Earl of Cathcart in 1814.