Sullivan is a city in Moultrie County, Illinois, United States. The population was 4,440 at the 2010 census. It is the county seat and largest city of Moultrie County. Sullivan is named after Sullivan's Island, South Carolina, where Fort Moultrie is located.
Sullivan was founded in 1845. Two years after Sullivan was founded, the first official courthouse of the county was built. It was a simple two-story brick building with a hipped roof, and the county jail was housed in the basement. The village would come alive with gossip when court was in session. Abraham Lincoln passed through this first courthouse many times from 1849-1852 as he practiced law in the Moultrie County circuit court. The present courthouse has a mural depicting this first courthouse.
Sullivan had not been a logical site for a county seat.