There are 4 US Post Offices in Grundy County serving a total of 10,056 residents equating to 2,514 residents per post office. It's estimated that approximately 16,050 packages are handled across the 4 county post offices annually. Grundy County is a county located in the U.S. state of Missouri. As of the 2010 census, the population was 10,261. Its county seat is Trenton. The county was organized January 2, 1841, from part of Livingston County, Missouri and named after U.S. Attorney General Felix Grundy."