There are 23 US Post Offices in Sangamon County serving a total of 164,317 residents equating to 7,144 residents per post office. It's estimated that approximately 262,268 packages are handled across the 23 county post offices annually. Sangamon County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 197,465. Its county seat is Springfield, the state capital. Sangamon County is included in the Springfield, IL Metropolitan Statistical Area."