York County is a county in the U.S. state of Pennsylvania. As of 2010, the population was 434,972. It is in the Susquehanna Valley, a large fertile agricultural region in South Central Pennsylvania. York County was created on August 19, 1749, from part of Lancaster County and named either for the Duke of York, an early patron of the Penn family, or for the city and shire of York in England. Its county seat is the city of York.