On February 16, 1843 Moultrie County was formed out of the northeastern portion of Shelby County and the southeast corner of Macon County. Macon and Piatt counties bound on the north, Douglas and Coles counties on the east, Shelby and Macon counties on the west, and Shelby County on the south.

The two significant names, Moultrie and Sullivan, so appropriately covering the county and one of the cities, represented two opposite sections of the country in the colonial days. General John Sullivan, born among the hills of New England and General William Moultrie who hailed from the Palmetto State, otherwise known as South Carolina. The love of liberty in both brought them into the service of the Continental Congress in the days of 1776. The distinction they gained in their military careers made their names revered and honored especially by the pioneers of Illinois who settled in this locality and named the county and town. The combination of these two names must have seemed very appropriate when they were selected as a recognition of the oneness and hopes of this new community.

When white man made their first appearance in the territory, around the seventeenth century, the main tribe in the area was the Illini, (Illinois) in French, which was a branch of the Algonquin family. In the eighteenth century, the Kickapoos and Pottawatomies began invading and taking over the area.

The first known white settler in Moultrie County was John Whitley. He settled at the point of timber, hence known as Whitley's Point, with his family, in the fall of 1826. Here he broke the first ground and erected the first cabin in what is now Moultrie County. The Whitley's came here from Kentucky and brought with them a number of fine Kentucky race horses. They were great horsemen and spent much time riding to neighboring settlements and matching races with other horsemen. In 1828 the Whitley family was joined by the Waggoner family who also settled near Whitley Creek. One of the sons of this family, John, later taught a school in the area. After moving to Sullivan, John took over the publication of the county's first newspaper, and served as county treasurer and circuit clerk.

The first election of officers for the County was in April 1843. John A. Freeland was named circuit clerk, county clerk and recorder; treasurer was Bennett B. Everett; sheriff was Isaac Walker, coroner was A.B. Lee and probate justice was David Patterson.

In 1845 Asa's Point won the majority vote, was named the permanent county seat and renamed Sullivan. Court was first held at James Camfield's cabin three miles southwest of Sullivan. In 1844, it was moved to Nelson (near present day Allenville) by a vote of the people.

The first Courthouse was completed March 6, 1848 and unlike the first courthouses in the older counties, this structure was built, not of logs, but of brick at a cost of $2,800. In 1849 a fence was put around the grounds to keep out the pigs, cows and horses that roamed the village.

This courthouse served the growing county for two decades. It was here that Stephen A. Douglas was scheduled to deliver a speech on October 20, 1858. Unbeknown to him, Abraham Lincoln was in Sullivan that day and accepted the invitation of his admirers to speak to them at Freeland Grove, outside the town. Lincoln was equally unaware of Douglas' presence. Each party arranged a parade, headed by its favorite. When the two leaders met near the center of town, trouble broke out among some of the more boisterous followers, resulting in a few battered heads before peace could be restored.

On November 25, 1864, the Courthouse burned destroying a large part of the county records. A new Courthouse was built with a fireproof vault in September 1866. By 1904 the County had outgrown that Courthouse and the present Courthouse was erected. It was dedicated November 12, 1906.