Website: www.lebanonil.org/city. Lebanon is a city in St. Clair County, Illinois, United States. The population was 5,523 at the 2010 census. Like many other places in Little Egypt or Southern Illinois, Lebanon was named after the Middle Eastern country of the same name. It is a part of the Metro-East region of the Greater St. Louis metropolitan area. Lebanon is home to McKendree University, the oldest college in Illinois.