There are 8 US Post Offices in Sierra County serving a total of 3,597 residents equating to 450 residents per post office. It's estimated that approximately 5,741 packages are handled across the 8 county post offices annually. Sierra County is a county located in the U.S. state of California. As of the 2010 census, the population was 3,240 making it the second-least populous county in California. The county seat is Downieville, and the only incorporated city is Loyalton. The county is located in the Sierra Nevada, northeast of Sacramento on the border with Nevada.