There are 11 US Post Offices in Mercer County serving a total of 15,085 residents equating to 1,371 residents per post office. It's estimated that approximately 24,077 packages are handled across the 11 county post offices annually. Mercer County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 16,434. Its county seat is Aledo. Mercer County is included in the Davenport-Moline-Rock Island, IA-IL Metropolitan Statistical Area."