New Castle County, Delaware On November 6, 1674, New Amstel was made dependent on New York Colony, and was renamed New Castle on November 11, 1674. On September 22, 1676, New Castle County was formally placed under the Duke of York's laws. It gained land from Upland County on November 12, 1678. On June 21, 1680, St. Jones County was carved from New Castle County. It is known today as Kent County, Delaware. On August 24, 1682, New Castle County, along with the rest of the surrounding land, was transferred from the Colony of New York to the possession of William Penn, who established
New Castle County, Delaware New Castle County, Delaware New Castle County is the northernmost of the three counties of the U.S. state of Delaware. As of the 2010 census, the population was 538,479, making it the most populous county in Delaware, with just under 60% of the state's population of 897,936 in the same census. The county seat is Wilmington. New Castle County is included in the Philadelphia-Camden-Wilmington, PA-NJ-DE-MD Metropolitan Statistical Area. The county is named after the English city of Newcastle. New Castle County has the highest population and population density of any Delaware county, and it is the smallest county in the