There are 8 US Post Offices in Marshall County serving a total of 11,954 residents equating to 1,494 residents per post office. It's estimated that approximately 19,080 packages are handled across the 8 county post offices annually. Marshall County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 12,640. Its county seat is Lacon. Marshall County is part of the Peoria, IL Metropolitan Statistical Area."