View from the southeast of the DeWitt County Courthouse, located at 201 W. Washington Street in Clinton, Illinois, United States. It was built in 1986. – Nyttend, Public domain – Wikimedia Commons DeWitt County is located in Illinois. About 16,247 people live here. Between 2010 and 2020, the population declined from 16,561 to 15,516. The area covers about 1,030.0 square kilometers. The region is divided into 7 Municipalities. The region was founded in 1839. The name originates from DeWitt Clinton (American politician) DeWitt County is county in Illinois, United States. More information at www.dewittcountyill.com.