He remained in this appointment until May 1918 when he was selected by General John J. Pershing, his classmate at West Point who was now in command of the AEF, to take over the 77th Division.[1][4] The first of the National Army divisions to arrive in France, the 77th began training with the British forces in northern France and Belgium where it soon gained a reputation as a first-class unit.[4]
After having been relieved over concerns about his physical condition, Duncan successfully convinced Pershing to return him to command. In October 1918, Duncan relieved Major General William P. Burnham as commander of the 82nd Division, and participated in the Meuse-Argonne Offensive.[3][4]
The President of the United States of America, authorized by Act of Congress, July 9, 1918, takes pleasure in presenting the Army Distinguished Service Medal to Major General George Brand Duncan, United States Army, for exceptionally meritorious and distinguished services to the Government of the United States, in a duty of great responsibility during World War I. Arriving in France with the first contingent of American troops, General Duncan commanded in turn a regiment, brigade, and division with conspicuous success. In the command of the 77th Division in the Baccarat sector his sound military judgment, energy, and resolution were important factors in the successes gained. Later, in command of the 82d Division in the Argonne-Meuse offensive, he proved himself a brilliant leader with great force and energy.[5]
Duncan married Mary Kercheval on October 23, 1895.[7] The couple had two sons: Daniel, born in 1901, and Henry, born in 1903. Daniel, however, died as a child in 1906.