Due to reclamation work, it is now joined physically to St. David's Island as a southeastern peninsula.
Its most notable feature is the expanse of Annie's Bay, which stretches along much of the island's east coast.
The island has been used by many United States government agencies, having been leased to the US Army, US Air Force and US Navy from 1942, during World War II, to 1995.
Swimming, walking, picnicking, bird watching and snorkelling are all permitted within the reserve.
With the adjacent Castle Islands Nature Reserve, Cooper's Island has been identified as an Important Bird Area by BirdLife International, principally because of its importance as a breeding site for seabirds.