As was typical of younger sons, Henry Hamilton entered the military. During the French and Indian War in North America, part of the Seven Years' War between Britain and France, he served as a Captain in the 15th Regiment of Foot. They fought in the 1758 attack on Louisbourg and the Battle of Quebec. With the support of Governor General of British North America Guy Carleton, Hamilton rose to the rank of brigade major. In 1775, he sold his commission, leaving the British Army for a political career.