Following the war, Gerow won promotion to permanent major in June 1920. Between the wars he commanded the Signal Corps (1919–1921) and alternated staff assignments in Washington; tours in Shanghai and the Philippines; and training courses at Fort Benning, Georgia; and Fort Leavenworth, Kansas, where he excelled. From 1935 onward, Gerow served in the War Department's War Plans Division. Promoted to brigadier general in October 1940, he became chief of the War Plans Division in December 1940. From December 1941, he served simultaneously as assistant chief of staff.
In February 1942, Gerow was promoted to major general and took command of the 29th Infantry Division, which began advanced training in Britain the following October. Gerow was a leading member of the talented group of top American officers that General Dwight D. Eisenhower gathered around himself when he was supreme commander in Europe. In July 1943, Gerow took command of V Corps of what became Lieutenant General Omar N. Bradley's First Army, which experienced heavy fighting in the Normandy Invasion of June and July 1944. On 25 August 1944, Gerow was the first Allied general to enter Paris. He then led V Corps in campaigns through northern France and the Rhineland. In January 1945, Gerow was promoted to lieutenant general and took command of the new Fifteenth Army, which secured the western French coast, taking the ports of Saint-Nazaire and Lorient. The area was also a staging ground for training and equipping units to join the 12th Army Group, which carried the battle into Germany itself.
From October 1945 to January 1948, Gerow was commandant of the Command and General Staff College. He then commanded the Second Army at Fort Meade, Maryland, retiring in July 1950. He died at Petersburg, Virginia, on 12 October 1972.
Ewing, Joseph. 29, Let's Go: A History of the 29th Infantry Division in World War II. Washington, DC: Infantry Journal Press, 1948.; Roy, Claude. Eight Days That Freed Paris. London: Pilot Press, 1945.; Weigley, Russell F. Eisenhower's Lieutenants: The Campaigns of France and Germany, 1944–1945. Bloomington: Indiana University Press, 1981.