In 1755, his cousin, Pierre Francois de Rigaud, Marquis de Vaudreuil-Cavagnal, put him in charge of building a fortress at the southern end of Lake Champlain. He spent several years there overseeing the construction of Fort Carillon (as it was called until the British captured it in the Battle of Ticonderoga in 1759 and renamed it Fort Ticonderoga). Although promoted to captain in 1757, he was refused the position of chief engineer (which he is often referred to as having been) of New France, a position which he had asked for after his father-in-law's death. The court instead appointed Nicolas Sarrebouce, an engineer in the French Army, who wasted no time in hindering Lotbinière's career, sending reports to Paris accusing him of incompetence and malfeasance, ruining his credibility with the Ministry of Marine. As some form of compensation his cousin, Governor-General Vaudreuil, gave him the seigneury of Alainville.