Carlyle is located in southwestern Illinois in central Clinton County (of which it is the county seat) about 45 miles east of St Louis between O'Fallon and Salem along U.S. Highway 50 on the banks of the Kaskaskia River near the southwestern corner of man-made Carlyle Lake, the largest artificial lake in the state. The population estimate for July 1, 1998 was 3,553, an increase of 79 since 1990.