In 1754, he was appointed chief engineer for General Braddock and went to America with the troops. 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). He spent most of the remainder of the French and Indian War in and around Albany, where his activities included the design and construction of numerous military fortifications, including a new fort on the site previously occupied by Fort William Henry, named Fort George. Montresor was promoted to Director and Lieutenant Colonel, Royal Engineers, 4 Jan. 1758.