His father, who belonged to a Huguenot refugee family, joined the British service and was naturalized, taking the name of Montresor.

Joining the Royal Artillery, Montresor is said to have been present at the 1727 siege of Gibraltar, but more credible accounts place him in Menorca, as a matross.

He is frequently said to have been present at the defeat of Braddock's expedition in 1755, but this appears to confuse him with his son John Montresor, who was wounded in the battle (some accounts suggest illness prevented the father's presence).

[2] By 1760, he had risen to the role of Chief Engineer in the Provinces, and in recognition of his efforts in New York he was granted 10,000 acres (40 km2) of land on the eastern (Vermont) side of Lake Champlain.

Throughout his career, he also drafted numerous maps and plans of the areas around which he was stationed, and kept journals, which have been published.