Joliet ( JOH-lee-et, -ET) is a city in Will and Kendall counties in the U.S. state of Illinois. Located 40 miles (64 km) southwest of Chicago, it is the county seat of Will County. Originally settled in 1852 as a steel mill town on the Des Plaines River, Joliet later became known for being the first major city outside of Chicago on U.S. Route 66. It is home to Old Joliet Prison, a historic decommissioned correctional facility that was the largest in the state at the time of its construction. The prison has been featured prominently in several films and television shows, including the 1980 film The Blues Brothers. As of the 2020 census, the population was 150,362, making it the third-most populous city in Illinois.