The city of Marion is the county seat of Williamson County, Illinois, United States. The population in Marion, IL was 16,855 according to the 202…The city of Marion is the county seat of Williamson County, Illinois, United States. The population in Marion, IL was 16,855 according to the 2020 census. It is part of a dispersed urban area that developed out of early 20th-century coal fields.