Early records describe as many as 12 mounds near Toolesboro. Subsequent settlement and excavation have reduced that number to seven. The family of George M. Mosier, a local land owner, donated a parcel of land containing the mound group to the state of Iowa. In 1966, the Toolesboro Mound Group was listed as a National Historic Landmark, and in 1969 a small visitor center and parking lot were built near the site. The state has designated the Historical Society of Iowa as custodian of the site.