Mound City was started by Edward C. McIntosh and his father, E.H. McIntosh, on June 10, 1884. It was named for the small hills just north of town, which they called bachelor mounds, and those mounds are still visible to the northwest. Mound City became part of Campbell County, Dakota Territory, five years before South Dakota became a state in 1889. In 1864, General Alfred Sully passed here during his military campaign against the Sioux. Just west of here, Lieutenant Colonel George A. Custer passed in 1873 on his way to Fort Abraham Lincoln near Bismarck. From there, he explored the Black Hills in 1874 and in 1876 mounted a military campaign in Montana that included the Battle of the Little Big Horn. In 1888, the county seat of Campbell County was moved from LaGrace, near the Missouri River, to Mound City, and Mound City remains the county seat today.