Abercromby served in the French and Indian War, and was promoted captain in 1761. On 30 Nov. 1775, he was promoted to lieutenant colonel of the 37th Regiment of Foot. During the American Revolutionary War, he fought at the Battle of Long Island, the Battle of Brandywine, the Battle of Germantown, the Battle of Crooked Billet, the Battle of Monmouth and at the sieges of Charleston and Yorktown, where he commanded the left wing of the British forces. He commanded a battalion of light infantry for most of the war.