Captain John Montresor (22 April 1736 – 26 June 1799) was a British military engineer in North America.
Stationed at Fort George (the former site of Fort William Henry) in 1765, he witnessed rioting in Albany and New York City in response to the Stamp Act, and in the same year was promoted to captain lieutenant, and engineer extraordinary, as well as barrackmaster for ordinance in North America. Over the next several years, he surveyed the boundary between New York and New Jersey, and he repaired or constructed barracks and fortifications in Boston, New York City, the Bahamas, and Philadelphia, where he would build a fortification on Mud Island. During this period he also took a six-month leave in England, and spent time on Bermuda. He also purchased an island in New York Harbor which would be called Montresor's Island (now Randall's Island).
He was in Boston at the outset of the American Revolutionary War, and marched with Percy to relieve the British troops returning from Concord. He was appointed chief engineer in America and captain in late 1775. He was present at the Battle of Long Island the next year, and was present at the execution of Nathan Hale on 22 September 1776. It is said that he kindly sheltered Hale in his office, giving him pen and paper to write final letters to his family, and that the execution moved him deeply. He was sent to the rebel lines under flag of truce to report the event, and he conveyed Hale's last words to William Hull. Having been superseded as chief engineer, he was placed as aide-de-camp on the staff of General William Howe, but was later reinstated as chief engineer. On 13 January 1777 his home on Montresor's Island was burned.
In 1777 he was involved in the military campaigns in New Jersey, and present at the action at Quibbleton. He also participated at Brandywine later that year, and accompanied the army to Philadelphia where he launched the attack that destroyed his own Mud Island defences. He directed the construction of new defences for the city, and along with John André, he was one of the planners of the lavish ball, the Mischianza, given in Philadelphia in honor of General Howe. Again superseded in his role as chief engineer, he returned to England and, in March 1779 resigned from the army, bringing to an end over two decades of American service, all reported in journals (although many of these were lost).