Resident Population in New Castle County, DE was 578.59200 Thous. of Persons in January of 2023, according to the United States Federal Reserve. Historically, Resident Population in New Castle County, DE reached a record high of 578.59200 in January of 2023 and a record low of 385.85600 in January of 1970. Trading Economics provides the current actual value, an historical data chart and related indicators for Resident Population in New Castle County, DE - last updated from the United States Federal Reserve on May of 2024.