The son of Charles Cathcart, 8th Lord Cathcart, and Marion Shaw, he was born on 21 March 1721. Opposed to the restoration of the Stuart monarchy, he became an aide-de-camp to the Duke of Cumberland and during the Battle of Fontenoy in 1745, was shot in the face. Joshua Reynolds' portrait (1753–55) shows the black silk patch he used to cover the scar on his cheek. This seemingly earned him the soubriquet 'Patch Cathcart'. The following year at the Battle of Culloden, again acting as ADC to Cumberland, Cathcart was once more wounded in battle.