John Forbes was born on September 5, 1707, at Dunfermline in Fife, Scotland, into a family of landed status. He was the son of Lieutenant Colonel John Forbes of Pittencrieff and Elizabeth Graham. His family connections linked him to Scotland’s military and professional classes, but his first intended career was medicine rather than the army. Forbes studied at the University of Edinburgh, one of Europe’s growing centers of medical learning, and received further medical training on the Continent. He qualified as a physician, an education that encouraged habits of observation, organization, and attention to physical conditions that later distinguished his military leadership.

In the 1720s Forbes entered the British Army as a surgeon. He subsequently exchanged medicine for a commissioned officer’s career, becoming a cornet in the Scots Greys in 1735. Cavalry service required knowledge of horses, transport, supply, and movement across difficult country. Forbes advanced to lieutenant in 1742 and captain in 1744. During the War of the Austrian Succession he served in Flanders and participated in major continental operations, including the Battle of Fontenoy in 1745.

Forbes also served during the Jacobite rising of 1745–1746. His Scottish background and knowledge of the Highlands made him useful to the government, while the campaign demonstrated the importance of roads, fortified posts, intelligence, and political relationships with local communities. By 1750 he had become lieutenant colonel of the Scots Greys. Fellow officers regarded him as methodical, intelligent, and unusually attentive to logistics and the welfare of his troops.

In 1757 Forbes was appointed colonel of the 17th Regiment of Foot and promoted to brigadier general in America. He arrived with a mature understanding that successful campaigning depended on more than battlefield courage. His medical training, cavalry experience, and knowledge of Scottish pacification all encouraged a deliberate approach based on secure communications, defensible staging points, and negotiation. Those qualities would shape his advance toward the Forks of the Ohio.