Aitkin County is a county located in the U.S. state of Minnesota. As of 2000, the population is 15,301. Its county seat is Aitkin. A portion of the Mille Lacs Indian Reservation is in the county. Aitkin County was established in 1857 as Aiken County and had a spelling change of its name to the current spelling in 1872. Aitkin County was named after William Alexander Aitkin, a fur trader for the American Fur Company, under John Jacob Astor. Aiken County originally consisted of the 17 townships closest to Mille Lacs Lake.