After 10 months of training the Division made ready to go overseas. Examined and reoutfitted at the staging area, Camp Merritt, New Jersey, the first elements sailed for France 6 July 1918. Most of the men landed in England, although a few were taken directly to France.

By 1 August the Infantry Brigades had been gathered at Montigny le Roi, and the Artillery Brigade at Camp de Souge and Clermont-Ferrand. At these places the men underwent a month of incessant drilling and long hours of marching, until they were declared ready for actual combat. On 29 August Major General William Johnston assumed command of the Division, and a very few days later, 7 September, it was assigned to the reserve of the First American Army during the St. Mihiel offensive, with headquarters at Sorcy.

 

 

When the success of the St. Mihiel offensive was assured, the 91st entered the Meuse-Argonne sector prepared to attack. Nearly every other Division employed in the Meuse-Argonne had previous combat experience. The 91st had had no such experience, yet it gave notable account of itself. On 25 September General John J. Pershing, Commander-in-Chief of the Allied Armies, personally visited Major General Johnston to express his confidence in the 91st before they marched into battle. The next day the Division showed that General Pershing's confidence had not been misplaced by breaking through two German lines and penetrating a third, advancing 8 kilometers. The enemy was driven from the strong points of Very, Epinonville, Gesnes, Eclis-fontaine, and Tronsol Farm. General George J. Cameron, Commanding General, Fifth Corps, paid high tribute to the Division in an order to General Johnston:

Despite the fact that this offensive was the Division's first entrance into combat, it captured more artillery, machine guns, and prisoners, and advanced a greater distance under fire than many Divisions with much longer combat experience. On 4 October, the Division was relieved by the 32nd Division and assembled near Contrisson.

One of the great honors given the Division came on 16 October, when, along with the 37th Division, it was named as part of the armies in Flanders, which, under King Albert, were about to launch the final crushing drive at the enemy in Belgium. The 91st attacked in the early morning mists of 31 October. From that time on until the very moment of surrender, 11 AM on 11 November, the Division drove the enemy back in panic. Although the enemy had been ordered to hold the heights between the Lys and the Excaut Rivers to the death, the 91st smashed them the first day, and by the evening of 1 November they were on the outskirts of Audenande. The next day the town was secured, and the Division pushed on to capture Welden, Petegem, and Kasteelwijk in rapid succession.

On the morning of 10 November, with the 182nd Infantry Brigade in the lead, the Division crossed the Scheldt River near Eyne. They drove forward through town after town, and had advanced beyond Moldergem when the order came to cease firing. In recognition of the superb courage and fighting ability the 91st Division had shown Major General De Goutte, who had resumed command of the Sixth French Army, issued an order which read, in part:

 

After the Armistice, elements of the 166th Field Artillery Brigade moved into Germany and occupied the village of Wittlich until February 1919. The infantry of the Division, after parading triumphantly with the men of the 37th before the cheering crowds of Brussels, patrolled the Franco-Belgian border west of Poperinghe from Beveren to Warande, for a short time. On 2 January the first contingent of men sailed for home, and echelons sailed thereafter as transportation became available. Division Headquarters, last to leave France, sailed 6 April, and final demobilization of the Division was completed at camps in California, Washington, and Wyoming by 14 May 1919.