James Moncrieff (sometimes spelled "Moncrief") was born in 1744 to James Moncrieff of Sauchop, Fife and his wife (name unknown). After graduating from the Royal Military Academy in 1762, he acted as an ensign for the British expedition to Havana. For the next 14 years he served in the Royal Engineers in the province of East Florida and in the West Indies. During that time, Moncrieff accumulated a substantial estate consisting of land holdings and slaves (primarily in Florida). He was promoted to sub-engineer and lieutenant on December 4, 1770, and then Captain on January 10, 1776.
Once the American Revolution began, Moncrieff acted as Captain in the Royal Engineers. He fought in the Brandywine battle on September 11, 1777, and was briefly captured by the Americans at Flatbush, New York in 1778. From July, 1778, until December 1782, Moncrieff served in the southern campaign, specifically in Georgia and South Carolina. He was promoted Brevet Major on December 27, 1779, after proving himself a highly efficient engineer during the Siege of Savannah. He organized fortifications and other military works which were integral to its successful defense. Moncrieff joined the expedition commanded by Henry Clinton to capture Charlestown, South Carolina in 1780. He was promoted to Lieutenant-Colonel on September 27, 1780. Following the success at Charlestown, James Moncrieff was put in charge of overseeing the defenses of the city (as Chief Engineer) where he remained until 1782.
The British Army evacuated Charlestown in December 1782, and Moncrieff returned to England. As a result of hostility from Americans, in 1784 he ordered his slaves in Florida to be removed to the Mosquito Coast, Nicaragua. When the British formally withdrew from the Coast in 1787, he requested that they be recovered to Jamaica. Only 51 of his slaves were actually able to be brought back to the West Indies due to unrest on the Mosquito Coast. Moncrieff remained in England, performing engineering tasks.
He received a promotion to Colonel in the army on November 18, 1790. In 1791, Moncrieff traveled to the West Indies, inspecting fortifications on various islands. Following the declaration of war between France and Great Britain, he was appointed quartermaster-general to the Duke of York (February 25, 1793), acting as de facto Chief Engineer. He assisted in British attacks along the northern French border and was promoted Regimental Lieutenant Colonel near the beginning of the siege of Valenciennes. James Moncrieff was killed in battle during the siege of Dunkirk in September 1793. He bequeathed the bulk of his estate to his daughter, Margaret Moncrieff.