Joliet, Illinois. Joliet (/ˈdʒoʊli.ɛt/ or /dʒoʊliˈɛt/) is a city in Kendall and Will counties in the U.S. state of Illinois, 40 miles (64 km) southwest of Chicago. It is the county seat of Will County. At the 2010 census, the city was the fourth largest in Illinois, with a population of 147,433.