There are 5 US Post Offices in Fairfield County serving a total of 23,337 residents equating to 4,667 residents per post office. It's estimated that approximately 37,248 packages are handled across the 5 county post offices annually. Fairfield County is a county located in the U.S. state of South Carolina. As of the 2010 census, its population was 23,956. Its county seat is Winnsboro. Fairfield County is part of the Columbia, SC Metropolitan Statistical Area.