There are 4 US Post Offices in Moultrie County serving a total of 12,871 residents equating to 3,218 residents per post office. It's estimated that approximately 20,544 packages are handled across the 4 county post offices annually. Moultrie County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 14,846. Its county seat is Sullivan. The name is pronounced as in \"mole tree\", unlike the pronunciation of its namesake, South Carolinian William Moultrie."