There are 6 US Post Offices in Jefferson County serving a total of 11,688 residents equating to 1,948 residents per post office. It's estimated that approximately 18,655 packages are handled across the 6 county post offices annually. Jefferson County is a county located in the U.S. state of Montana. As of the 2010 census, the population was 11,406. Its county seat is Boulder. The county was created in 1865 and named for President Thomas Jefferson. Jefferson County is part of the Helena, MT Micropolitan Statistical Area."