Sullivan was founded in 1845 as Asa's Point. 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 to 1852, as he practiced law in the Moultrie County circuit court. The present courthouse (the county's third) contains a mural depicting this first courthouse.
In the opinion of early local leaders, Sullivan was not a logical site for a county seat. The village of Nelson (which no longer exists) had already been developed, and the prairie on which Sullivan would be built lacked proper drainage and was a breeding ground for malaria-carrying mosquitoes. In 1844, however, it was determined that centrally located Sullivan was to be the county seat.