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. Today Marion serves as the largest retail trade center in Southern Illinois with its central location along Interstate 57 and Illinois Route 13. It was dubbed the "Hub of the Universe" by former mayor Robert L. Butler.