Chillicothe is a city on the Illinois River in Peoria County, Illinois, United States. The population was 6,097 at the 2010 census. Chillicothe is j…Chillicothe is a city on the Illinois River in Peoria County, Illinois, United States. The population was 6,097 at the 2010 census. Chillicothe is just north of the city of Peoria and is part of the Peoria Metropolitan Statistical Area.