There are 3 US Post Offices in Massac County serving a total of 14,935 residents equating to 4,978 residents per post office. It's estimated that approximately 23,838 packages are handled across the 3 county post offices annually. Massac County is a county located in the state of Illinois. According to the 2010 census, it had a population of 15,429. Its county seat is Metropolis. Massac County is included in the Paducah, KY-IL Micropolitan Statistical Area. It is located along the Ohio River, in the portion of the state known locally as \"Little Egypt\"."