Originally, in 1909, what is today Fort McCoy was the Sparta Maneuver Tract, mainly intended for field artillery training, was divided on the local rail line into two camps, but in short order the entire post was unified as one post known as Camp Bruce E. McCoy. McCoy was a Civil War Union Army captain and the father of Major General Robert McCoy who had owned much of the camp land. In 1926 the name was changed to honor the Major General and not until 1974 was the post name changed to Fort Robert McCoy.


Fort McCoy expanded modestly in the years before World War I, and greatly during the war, adding rifle ranges, administration buildings, storehouses, stables, barracks, mess halls, and the other requirements of a standing post. After World War I the post was reassigned as the Sparta Ordnance Depot, and thousands of shells and tons of powder and gun cotton were stored here for shipping. From 1923 to 1926 the Sparta Ordnance Depot turned to converting the unused explosives to dynamite, and much of the camp was disassembled to make packing crates, under administration of the Department of Agriculture. At the time, the greatest war in history was in the past and no future wars were expected; dynamite was needed for industry.


In 1926 the camp was returned to the US Army, and renamed as Camp Robert McCoy. The camp was once again used for artillery training and Reserve and National Guard training. In 1933 the Fort McCoy grounds were the site of a Civilian Conservation Corps camp, providing shelter and work and a small income for working men, considered better than having bands of the starving homeless unemployed drifting through America. Many of these CCC men, under regimented camp life, later went through Army training fairly smoothly.


World War II brought a new round of construction to McCoy; the camp was modernized and expanded as part of the pre-arming of the United States. Artillery systems were also upgraded; the last horse-drawn artillery was not retired at McCoy until 1940. This put the US Army ahead of the curve; the Germans were still using horse-drawn artillery in Europe until the end of the war.


The CCC camp at McCoy was the site of a POW camp during World War II, for prisoners from the North African, European, and Pacific theaters, and also an internment camp for Japanese-Americans.


McCoy is where the 442nd Infantry Regimental Combat Team and 100th Infantry Battalion, units of Japanese-Americans from the Hawaiian National Guard, who later went on to a legendary campaign in the Italian theater. The 442 was the most decorated unit in US Army history, achieving a remarkable twenty-one Medals of Honor, fifty-two Distinguished Service Crosses, one Distinguished Service Medal, five hundred sixty Silver Stars (twenty-eight with oak clusters), and thousands of other awards. McCoy also trained many other units, including nurses, Limited Service Soldiers (disabled soldiers still able to contribute to the war effort in specialist fields), and a radio school.


At the end of the war McCoy was one of many intake and training centers to convert to a separation center, and thousands of soldiers were out-processed to the northern Midwest here. In 1947 the camp was inactivated, although still used by Reserve and National Guard training. In 1950 McCoy was reactivated as a training center for soldiers going to the Korean War, and then again used as a separation center and once again returned to various other government activities, including job corps training.


In 1974 Fort McCoy became a permanent installation, with a year-round training mission, continuing its support activities for other agencies. In 1980 Fort McCoy became a relocation camp for Cuban refugees after a massive migration from communist Cuba. Through the 1980s and 1980s Fort McCoy increased its training facilities and complement, becoming today one of the most prominent Army posts in the north-central US.