During WWII, its first assignment was to the British Expeditionary Force. Along with virtually the whole of that force, it was evacuated from Dunkirk after the Germans broke through Allied lines. I Corps then remained in the United Kingdom until the landings in Normandy for Operation Overlord. It was assigned to 2nd Army in 21st Army Group for most of the campaign in France, the Low Countries and Germany.
After the defeat of Germany, 21st Army Group became the British Army of the Rhine, and I Corps became the principal combat element of the BAOR. The corps remained active in Germany until the 1993 defence cuts.