Aitkin County, Minnesota. Aitkin County (/ˈeɪkɪn/ AY-kin) is a county located in the U.S. state of Minnesota. As of the 2010 census, the population was 16,202.[2] Its county seat is Aitkin.[3] A portion of the Mille Lacs Indian Reservation is in the county. The county was created in 1857 and organized in 1871.