Physical Characteristics. Nassau County is in northeast Florida and is bordered by Georgia, the Atlantic Ocean, and Duval and Baker counties. The county has 649.04 square miles of land and 21 square miles of water for a total of 670.04 square miles. The average January temperature is 55.6 degrees fahrenheit, and the average August temperature is 81.5 degrees fahrenheit. The average annual rainfall is 48.50 inches.
History. Nassau County was established from St. Johns County in 1824 and named for Nassau Sound which separates Nassau and Duval counties. The name comes from the capital of the Bahamas, which is named for the Duchy of Nassau, a former German state.
