There are 1 US Post Offices in Clark County serving a total of 860 residents equating to 860 residents per post office. It's estimated that approximately 1,373 packages are handled across the 1 county post offices annually. Clark County is a rural county in the U.S. state of Idaho. As of the 2010 Census the county had a population of 982, making it the least populous county in Idaho. The county seat and largest city is Dubois. The county was established on February 1, 1919, partitioned from Fremont County. It was named for State Senator Sam K. Clark, an early pioneer on Medicine Lodge Creek."