On 11 June 1744 Abercrombie was made Lieutenant of the 1st Foot. On 16 February 1756, he was promoted to the rank of Captain of the 42nd Foot. With this rank he served in the French and Indian War, notably as one of General Abercrombie's aides in the Battle of Fort Carillon at Ticonderoga in 1758 before being made aide-de-camp to General Amherst in 1759. He was promoted to the rank of lieutenant colonel in 1770.