XXI Corps fought for 116 days in the European Theater of Operations, starting in the Alsace, crossing into southern Germany, and swarming into Austria, with individual elements reaching into northern Italy.

From 25 January until 16 February 1945, XXI Corps was attached to the French First Army and took part in bitter winter combat that ultimately collapsed the Colmar Pocket.

After a period of rest, the corps returned to the front on 28 February 1945 and pushed to the edge of the Siegfried Line during the first week of March, 1945.

On 3 May 1945, units of the corps that included the 12th Armored Division entered Austria via Kufstein,[2] and advanced along the Inn River as far as Wörgl until met the next day by troops of the 409th Infantry Regiment of the Fifth Army's 103rd Infantry Division of VI Corps radiating northeast from Innsbruck.[1] German forces in the area unconditionally surrendered on 6 May 1945.

The post-Second World War activation and inactivation occurred at Indiantown Gap Military Reservation, Pennsylvania.