Pennyhill is an unincorporated community in New Castle County, Delaware, United States. New Castle County is the northernmost of the three counties of the U.S. state of Delaware. As of 2010 its population was 538,479, an increase of 7.6% over the previous decade. The county seat is Wilmington. The center of population of Delaware is located in New Castle County, in the town of Townsend. It is the most affluent of the three counties in the state of Delaware. In addition it is the smallest in area but largest in population.