There are 3 US Post Offices in Mercer County serving a total of 4,037 residents equating to 1,346 residents per post office. It's estimated that approximately 6,444 packages are handled across the 3 county post offices annually. Mercer County is a county located in the northwestern portion of the U.S. state of Missouri. As of the 2010 census, the population was 3,785, making it the second-least populous county in Missouri. Its county seat is Princeton. The county was organized February 14, 1845 and named for General John F. Mercer of the Revolutionary War."