Herrin is a city in Williamson County, Illinois. The population was 12,352 at the 2020 census. The city is part of the Marion-Herrin Micropolitan Ar…Herrin is a city in Williamson County, Illinois. The population was 12,352 at the 2020 census. The city is part of the Marion-Herrin Micropolitan Area and is a part of the Carbondale-Marion-Herrin, Illinois Combined Statistical Area with 123,272 residents, the sixth most populous Combined statistical area in Illinois.