Sullivan is the largest city and the county seat of Moultrie County, Illinois, United States. The population was 4,413 at the time of the 2020 cens…Sullivan is the largest city and the county seat of Moultrie County, Illinois, United States. The population was 4,413 at the time of the 2020 census. Sullivan is named after Sullivan's Island, South Carolina, where Fort Moultrie is located.
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