John Campbell was born at Loudoun Castle, near Galston. A professional soldier, he raised a regiment of Highlanders to fight the Jacobites during the 1745 Rising. He served as adjutant-general to Sir John Cope, commander-in-chief of the government forces. At the battle of Prestonpans parts of his regiment were captured. Loudoun subsequently went north to Inverness to raise another force and took part in a number of further operations against the Jacobites. In February 1746 his attempt to capture Prince Charles at Moy Hall ended in failure. Later in his career he became commander-in-chief of the British forces in North America, serving against the French with little success. During the Seven Years' War Loudon commanded the British troops in Portugal against Spain.