There are 20 US Post Offices in Mercer County serving a total of 118,405 residents equating to 5,920 residents per post office. It's estimated that approximately 188,988 packages are handled across the 20 county post offices annually. Mercer County is a county located in the U.S. state of Pennsylvania. As of the 2010 census, the population was 116,638. Its county seat is Mercer, and its largest city is Hermitage. The county was created in 1800 and later organized in 1803. Mercer County is included in the Youngstown-Warren-Boardman, OH-PA Metropolitan Statistical Area."