Wilmington is a city in Will County, Illinois, United States. Located on Illinois Route 53 and Historic U.S. Route 66 along the east bank of the Kankakee River, it is approximately 60 miles south-west from downtown Chicago. The population was 5,664 at the 2020 census. Wilmington is just south of Midewin National Tallgrass Prairie, which is home to the largest bison herd in the state.