The 91st Division has a proud history of military service dating back to World War 1. Constituted on 5 August 1917 at Camp Lewis, Washington, the Division soon thereafter departed for England in the summer of 1918. In September 1918, the Division's first operation was in the St. Mihiel Offensive in France. Serving under the U.S. Army's V Corps, the Division fought in the Meuse-Argonne Offensive and successfully helped to destroy the German First Guard Division and continued to smash through three successive enemy lines.

Twelve days before the end of WWI, the Division, as part of the VII Army Corps, helped drive the enemy east across the Escaut River. The Division was awarded separate campaign streamers for its active role in the Lorraine, Meuse-Argonne and Ypres-Lys campaigns. In 1919, the 91st was deactivated at the Presidio of San Francisco.

After being reconstituted in 1921 as part of the Organized Reserves, the Division then served as an administrative control center for the next 21 years.

As the first battles of World War 11 were being fought in 1942, the Division was reactivated at Camp White, Oregon. Once again, the Division departed for the European Continent. There, the 361st Regimental Combat Team was detached to participate in the battles for Rome and the Arno River. It became the first V Army unit to reach the river. In September, 1944, the Division crossed the Sieve River, outflanked the famous Gothic Line, and captured the Futa Pass. For its part in combat, the Division was awarded the North Apennines, Po Valley and Rome-Arno campaign streamers. The division was deactivated at Fort Rucker, Alabama, in December 1945.

In December 1946, the 91st was reactivated at the Presidio of San Francisco as part of the U.S. Army Reserve. In 1959, the Division was reorganized and redesignated as the 91st Division (Training). In 1993 the Division was again reorganized and redesignated as the 91st Division (Training Support).