City Island is about 1.5 mile long by .5 mile wide. Once attached to the town of Pelham, in Westchester County, it is now part of the New York City Borough of The Bronx. Its land area is .395 sq mi, or 252 acres. The island belongs to Bronx Community Board 10.
City Island is at the extreme western end of Long Island Sound, south of Pelham Bay and east of Eastchester Bay. The body of water between City Island and Hart Island to the east is known as City Island Harbor.