On 21 September 1917, the division arrived at St Nazaire, France. It was second division of the American Expeditionary Force to arrive into the theater at the time, and the first division wholly organized in the United States, joining the 1st Infantry Division. Two additional divisions completed the first wave of American troop deployment, with the 2nd Division formed in France and the 42nd Division arriving at St. Nazaire on 29 October. The division immediately moved to Neufchâteau for training, as most of the division's soldiers were raw recruits, new to military service. Because of this, much of the division's force was trained by the experienced French forces. It trained extensively with the other three US divisions, organized as the U.S. I Corps in January 1918, before being moved into a quiet sector of the trenches.
The 26th Infantry Division remained in a relatively quiet region of the lines along the Chemin des Dames for several months before it relieved the 1st Division near St. Mihiel on 3 April. In late April, German infantry conducted a raid on positions of the 26th Division, one of the first attacks on Americans during the war. At 0400 on 20 April, German field artillery bombarded the 102nd Infantry's positions near Seicheprey before German stoßtruppen moved against the village. The artillery box barrage, continuing 36 hours, isolated American units. The Germans overwhelmed a machine gun company and two infantry companies of the 102nd and temporarily breached the trenches before elements of the division rallied and recaptured the village. The Germans withdrew before the division could counterattack but inflicted 634 casualties, including 80 killed, 424 wounded, and 130 captured, while losing over 600 men, including 150 killed of their own. Similar raids struck the 101st infantry at Flirey on 27 May, and the 103rd Infantry at Xivray-et-Marvoisin on 16 June, but were repulsed. The 26th Division was relieved by the 82nd Division on 28 June, moved by train to Meaux, and entered the line again northwest of Chateau Thierry, relieving the U.S. 2nd Division on 5 July.
As the size of the American Expeditionary Force grew, the division was placed under command of I U.S. Corps in July. When the Aisne-Marne campaign began shortly thereafter, the division, under I U.S. Corps was placed under command of the French Sixth Army protecting its east flank. When the offensive began, the division advanced up the spine of the Marne salient for several weeks, pushing through Belleau Wood, moving 10 miles from 18 to 25 July. On 12 August it was pulled from the lines near Toul to prepare for the next offensive. The division was then a part of the offensive at St. Mihiel, during the Battle of Saint-Mihiel. The division then moved in position for the last major offensive of the war, at Meuse-Argonne. This campaign was the last of the war, as an armistice was sighed shortly thereafter. During World War I the division spent 210 days in combat, and suffered 1,587 killed in action and 12,077 wounded in action. The division returned to the United States and was demobilized on 3 May 1919 at Camp Devens, Massachusetts.
In the years following World War I, the division remained in the National Guard, seeing periodic reorganizations but no major deployments except for weekend training. In 1921, the 102nd Infantry Regiment was replaced in the guard by the 182nd Infantry Regiment. In 1923, the 103rd Infantry Regiment was replaced with the 181st Infantry Regiment. In 1941, the 101st Infantry Regiment was replaced with the 164th Infantry Regiment briefly; one year later it was relieved from the division, along with the 182nd Infantry Regiment, in order to form the Americal Division.