According to the U.S. Census Bureau, the county has a total area of 453.2 square miles (1,174 km2), of which 284.7 square miles (737 km2) is land and 168.5 square miles (436 km2) (37%) is water.[39]

Nassau County borders the Long Island Sound on the north and the open Atlantic Ocean on the south. The county occupies a portion of Long Island immediately east of the New York City borough of Queens. It is divided into two cities and three towns, the latter of which contain 64 villages and numerous hamlets. The county borders Connecticut across the Long Island Sound.

Between the 1990 U.S. census and the 2000 U.S. census, the Nassau County exchanged territory with Suffolk County and lost territory to Queens County.[40] Dozens of CDPs had boundaries changed, and 12 new CDPs were listed.[40]