The post has been in virtually constant use since it was first formed as the "Sparta Maneuver Tract" on 14,000 acres (57 km²) in 1909. At first the tract was made up of two camps, Camp Emory Upton and Camp Robinson. These were separated by a line of the Chicago, Milwaukee, St. Paul and Pacific Railroad that ran across the land from east to west. In 1910, the army renamed the entire tract "Camp Bruce E. McCoy" for Robert Bruce McCoy, a retired major general, who first proposed the area as a training ground and bought part of the property on which the fort stands. In 1926 the name of the post was shortened to "Camp McCoy".
In 1938, the United States began a major expansion of the camp. This included the addition of over 45,000 acres (180 km²) to the post, as well as the construction of several new structures such as living quarters for the troops. This increased the camp's capacity to 35,000 soldiers. In all, the project was estimated to have cost about $30 million. The expansion was officially concluded with a new inauguration on August 30, 1942. During World War II, Camp McCoy was used as a training facility for units from across the United States that were preparing to enter combat. The post was also used as a Prisoner Of War (POW) camp during the conflict. The camp was briefly deactivated following World War II, but with the advent of the Korean War in 1950, it was once again used for training. This continued until 1953, when the camp was again deactivated. It was then used to house various small national, state and civilian projects, and served as a training center for the National Guard and the Job Corps. In the 1970s, a variety of ideas were offered to make use of the camp's land. The ideas included a suggestion by researchers to balance Wisconsin's population distribution by creating a major city on the 60,000 acre (240 km²) post that would rival Milwaukee. In response, a Milwaukee official proposed that the camp be used as a landfill for Milwaukee garbage.
In 1973, the Army reactivated Camp McCoy as a permanent training center, and on September 30, 1974, it was officially redesignated as Fort McCoy. In the 1990s, a second major construction project was undertaken, costing around $140 Million. Today Fort McCoy serves as a Total Force Training Center. Around 100,000 members of the military are trained at the fort every year , and the total number has exceeded 149,000 in the past.