The large number of troops assigned to the American Expeditionary Forces (AEF) required the activation of subordinate commands.

First Army, now under Lieutenant General Hunter Liggett, was inactivated on April 20, 1919, five months after the Armistice with Germany which ended hostilities.

Headquarters, First Army, was activated on 8 September 1932 at Governor’s Island, New York, and assumed control over the First, Second, and Third Corps Areas.

First Army's first commander, from 1932 to 1936, was Major General Dennis E. Nolan, who had been the American Expeditionary Force's (AEF) chief of intelligence during World War I.

He was followed by Major General Fox Conner, previously First Corps Area commander, who had been the AEF's Chief of Operations in World War I.

In the years between the wars, Conner was a crucial mentor in the careers of Dwight Eisenhower and George C. Marshall.

It began to establish and develop its own staff and participated in the large-scale Army maneuvers in Louisiana and North Carolina between 1939 and 1941.

First Army's entry into World War II began in October 1943 as Bradley returned to Washington, D.C., to receive his command and began to assemble a staff and headquarters to prepare for Operation Overlord, the codename assigned to the establishment of a large-scale lodgement on the European Continent following Operation Neptune, which was the invasion of Normandy.

The two American airborne divisions that landed, the 82nd and 101st, were scattered all over the landscape, and caused considerable confusion among the German soldiers, as well as largely securing their objectives, albeit with units completely mixed up with each other.

Once the beachheads were linked together, its troops struck west and isolated the Cotentin Peninsula, and then captured Cherbourg.

In Operation Lumberjack, First Army closed up to the lower Rhine by 5 March, and the higher parts of the river five days later.

In May 1945, advance elements of First Army headquarters had returned to New York City and were preparing to redeploy to the Pacific theater of the war to prepare for Operation Coronet, the planned second phase of Operation Downfall the proposed invasion of Honshū, the main island of Japan in the spring of 1946, but the Japanese surrender in August 1945 thanks to the atomic bombings of Hiroshima and Nagasaki terminated that effort.

In a 1993 reorganization, five divisions carried out that training and support mission: In 1993, Headquarters First Army relocated to Fort Gillem, near Atlanta, Georgia, and became responsible for the training and mobilization of all Army Reserve and National Guard units in the United States and providing assistance to the civilian sector during national emergencies and natural disasters.

In the latter role, First Army's contributions during the 2005 Hurricane Katrina disaster was a rare bright spot in leading federal relief efforts in the aftermath of the storm.

Its commander, Russel L. Honoré, a Louisiana native, became a nationally recognized figure in his direct, no-nonsense approach to disaster relief which earned First Army a Joint Meritorious Unit Award.

In 2005, a Base Realignment and Closure (BRAC) Commission decision called for the relocation of First Army headquarters to Rock Island Arsenal, Illinois, in 2011.

The Army Reserve mobilizes Focused readiness units (FRU) to meet Operational plan (OPLAN) requirements of the combatant commander (CCDR).