The combined value of land and rural real estate for sale in Williamson County, Illinois, recently on Land.com was around $58 million. This represents nearly 2,000 acres of land for sale in Williamson County. Williamson County ranks 14th out of 102 counties in the Inland Empire for the total number of acres of land currently advertised for sale. Among cities in Williamson County with land and rural real estate for sale, Marion had the most land for sale. Government and state-level services are the main economic drivers in Williamson County. Williamson County is the 65th biggest county (444 square miles) in Illinois. It's located in the Southern Illinois region of the state.