Lebanon is a city in St. Clair County, Illinois, United States. The population was 4,691 at the 2020 census It is a part of the Metro East region of t…Lebanon is a city in St. Clair County, Illinois, United States. The population was 4,691 at the 2020 census 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.