The land area is 8.687 sq. mi. (22.5 km squared), and a permanent population of 491 people was reported as of the 2000 census. (There are hundreds of thousands of summertime residents, groupers and daytrippers.) The stated land area and population figures result when this section is subtracted out.

Fire Island is about 5½ miles (8.9 km) south of the main land of Long Island (although within a mile in its eastern portion). It is separated from the main land by a series of interconnected bays: Great South Bay, Patchogue Bay, Bellport Bay, Narrow Bay, and Moriches Bay. The island has very limited access by automobile for day use, from Long Island by the Robert Moses Causeway on its western end to Robert Moses State Park and by William Floyd Parkway (Suffolk County Road 46) to Smith Point County Park near its eastern end. Motor vehicles are not permitted on the approximately 20 miles in between (except for utility, construction and emergency access and with limited beach driving permits in winter). Essentially the island and its resort towns are accessible only by the numerous ferries.

Fire Island is located at 40° 39' 35" North, 73° 5' 23" West (40.653188, -73.125795). According to the United States Census Bureau, Fire Island has a total area of 8.742 mi squared (22.64 km squared), which includes 0.1415 km squared of water.

The physical attributes of the island have changed over time and it continues to change. At one point it stretched more than 60 miles (97 km) from Jones Beach Island to Southampton.

Around 1683, Fire Island Inlet broke through, separating it from Jones Beach Island.

The Fire Island Inlet was to grow to nine miles (14 km) in width before receding. The Fire Island Lighthouse was built in 1858, right on the inlet, but Fire Island's western terminus at Democrat Point has steadily moved west so that the lighthouse today is six miles (10 km) from the inlet.

Fire Island separated from Southampton in a 1931 Nor'easter when Moriches Inlet broke through. Moriches Inlet and efforts by local communities east of Fire Island to protect their beach front with jetties have led to an interruption in the longshore drift of sand going from east to west and is blamed for erosion of the Fire Island beachfront. Between these major breaks there have been reports over the years of at least six inlets that broke through the island but have since disappeared.

The origin of Fire Island's name is not certain. It is believed its Native American name was Sictem Hackey, which translated to "Land of the Secatogues". The Secatogues were a tribe in the Bay Shore, New York, area. It was part of what was also called the "Seal Islands".

Other versions say the island derived its name from fires built on the sea's edge by Native Americans or by pirates to lure unsuspecting ships into the sandbars. Some say it is how portions of the island look to be on fire from sea in the Autumn. Yet another version says it comes from the rash caused by poison ivy on the island.

Under Public Law 88-587, Fire Island National Seashore (FIIS) was established on September 11, 1964, "for the purpose of conserving and preserving for the use of future generations relatively unspoiled and undeveloped beaches, dunes and other natural features."

The character, or physiognomy, of Fire Island is typical of Atlantic barrier islands that grade from a primary dune along the ocean to salt marsh along the bay. The dominant vegetation includes pitch pine (Pinus rigida), beach grass (Ammophilia breviligulata), wax myrtle (Myrica cerifera), bayberry (M. pensylvanica), shadbush (Amelanchier canadensis), and common greenbrier (Smilax rotundifolia). This particular composition of vegetation is typical of the island except within the various communities where residents have planted non-indigenous vegetation.

The percentages of terrestrial habitats found at Fire Island National Seashore include: 10% forested and 40% wetlands, 25% open (beach, swale and fields), 25% developed either by the National Park Service or the 17 local communities on the island. Of the submerged portion, 80% is in the Great South Bay and 20% is the Atlantic Ocean. All existing habitats within the Seashore are listed as threatened.

Unique resources include the Sunken Forest, a federal wilderness area (520 hectares), and eel grass beds. The Sunken Forest on Fire Island is a 16 hectare maritime oak-holly forest occurring behind the secondary dune, one of only a few mature maritime forests in the New York area and the northernmost holly-dominated maritime forest on the Atlantic barrier island chain. The Nature Conservancy listed this community type as globally imperiled (G2), and in 2001 the New York Natural Heritage Program ranked this maritime holly forest as "globally rare" or "G1G2 S1" meaning there are few remaining occurrences of this assemblage of plants throughout the world. Both federal and New York State endangered species either breed or germinate in the park, along with eleven other species of concern.