During the French and Indian War, he was selected by the New York Assembly, with the support of his brother James, then acting Governor, to provide provisions for New York provincial units. During the war, De Lancey commanded the New York Provincial Militia, 1755-1763, and commanded a provincial detachment in the Ticonderoga campaign of 1758. In 1766, De Lancey was one of the judges in the Pendergast case, where the alleged leader of the Dutchess County land rebels was convicted and sentenced to death.