There are 3 US Post Offices in Granite County serving a total of 2,710 residents equating to 903 residents per post office. It's estimated that approximately 4,325 packages are handled across the 3 county post offices annually. Granite County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 3,079. Its county seat is Philipsburg. The county was founded in 1893 and named for a mountain which contains the Granite Mountain silver mine."