There are 10 US Post Offices in Missoula County serving a total of 76,264 residents equating to 7,626 residents per post office. It's estimated that approximately 121,726 packages are handled across the 10 county post offices annually. Missoula County is a county located in the State of Montana. As of the 2010 census, the population was 109,426, making it the second-most populous county in Montana. Its county seat and largest city is Missoula. The county was founded in 1860. Missoula County comprises the Missoula, MT Metropolitan Statistical Area."