Painted by John Singleton Copley, circa 1771. Montresor became the British Army's Chief Engineer in North America in 1775, succeeding his father James Montresor.

John Montresor was born in Gibraltar in 1736 to James Montresor, a career army officer, and Mary Montresor (nee Haswell). John himself married in 1764 to Francis Tucker, the daughter of a British army lieutenant.

Montresor began his service in America as an engineering assistant to his father. John would become a principle resource for fortifications for the British colonies there, and was invited to Philadelphia in 1771 to meet with the provincial government to discuss the need for a fort along the Delaware River. The original mission of the fort was to discourage pirates and privateers.

Montresor offered several designs, each in turn declined as too expensive until Montresor submitted a design he considered clearly inferior but inexpensive. After this final submission, he left for New York (presumeably in disgust), not staying to help start the construction.

The fort's construction dragged on for years, suffering several work stoppages as funding dried up. As war with Britain broke out in 1775, Philadelphia renewed her interest in river defense and construction work was begun again. But again, the improvements came in spurts as military priorities were adjusted and readjusted.

The autumn of 1777 brought Capt. John Montresor back to Philadelphia, with General Howe's army; and this army occupied Philadelphia from September of that year to May of the next. To secure a more reliable source of supply, the British army needed to link up with the navy; and to accomplish this clearing the river of impediments was a necessity.

To this end, Howe employed Capt. Montresor to construct platforms and defensive embankments for artillery batteries on Province Island near Fort Mifflin. It is from these batteries that Fort Mifflin was subjected to the largest bombardment of the war.