Rock Hill is a suburban town in St. Louis County, Missouri, United States. The population was 4,635 at the 2010 census. It is home to the Fairfa…Rock Hill is a suburban town in St. Louis County, Missouri, United States. The population was 4,635 at the 2010 census. It is home to the Fairfax House on the National Register of Historic Places. According to the United States Census Bureau, the city has a total area of 1.09 square miles, all land.