There are 7 US Post Offices in Grundy County serving a total of 11,532 residents equating to 1,647 residents per post office. It's estimated that approximately 18,406 packages are handled across the 7 county post offices annually. Grundy County is a county located in the U.S. state of Tennessee. As of the 2010 census, the population was 13,703. Its county seat is Altamont. The county is named in honor of Felix Grundy."