There are 6 US Post Offices in Jefferson County serving a total of 17,632 residents equating to 2,939 residents per post office. It's estimated that approximately 28,143 packages are handled across the 6 county post offices annually. Jefferson County is a county located in the U.S. state of Iowa. As of the 2010 census, the population was 16,843. The county seat is Fairfield. The county was formed on January 21, 1839 and named after President Thomas Jefferson. Jefferson County comprises the Fairfield, IA Micropolitan Statistical Area.