John Campbell was born on May 5, 1705, the only son of Hugh Campbell, 3rd Earl of Loudoun, and Lady Margaret Dalrymple. He inherited connections to two influential Scottish families: the Campbells, long prominent in politics and war, and the Dalrymples, a major legal and governmental dynasty. His father supported the Hanoverian succession and took part in suppressing the Jacobite rising of 1715. John therefore grew up in a household where loyalty to the new dynasty, Scottish aristocratic responsibility, and military service were closely intertwined.

Campbell entered the British Army while young and succeeded his father as 4th Earl of Loudoun in 1731. As a Scottish representative peer, he also took a place in national politics. His military advancement combined rank, family standing, and active service. He became governor of Stirling Castle and served as aide-de-camp to King George II. During the War of the Austrian Succession he campaigned with British forces on the Continent and was present at the Battle of Dettingen in 1743.

The Jacobite rising of 1745 gave Loudoun a more demanding independent role. He raised a regiment of Highlanders loyal to the government and operated in northern Scotland against the forces of Charles Edward Stuart. His troops gathered intelligence, guarded strategic positions, and attempted to mobilize anti-Jacobite clans. Loudoun was surprised and defeated at Moy in 1746, but he escaped and continued serving the government. The campaign taught him the difficulties of irregular warfare, divided local loyalties, and military administration across a rugged region.

After the rising, Loudoun continued to receive important appointments and became colonel of the 30th Regiment of Foot. He was regarded as conscientious and hardworking, though formal, cautious, and strongly committed to military hierarchy. In 1756 the ministry appointed him commander in chief in North America and governor general of Virginia. He crossed the Atlantic with expectations that he would impose unified direction on colonies accustomed to controlling their own troops, money, and quarters.