There are 6 US Post Offices in Lewis County serving a total of 9,783 residents equating to 1,631 residents per post office. It's estimated that approximately 15,615 packages are handled across the 6 county post offices annually. Lewis County is a county located in the northeastern portion of the U.S. state of Missouri. As of the 2010 census, the population was 10,211. Its county seat is Monticello. The county was organized January 2, 1833 and named for Meriwether Lewis, the explorer and Governor of the Louisiana Territory. Lewis County is part of the Quincy, IL\u2013MO Micropolitan Statistical Area."