There are 9 US Post Offices in Piatt County serving a total of 15,827 residents equating to 1,759 residents per post office. It's estimated that approximately 25,262 packages are handled across the 9 county post offices annually. Piatt County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 16,729. Its county seat is Monticello. Piatt County is part of the Champaign\u2013Urbana, IL Metropolitan Statistical Area."