The Dry Tortugas are low coral and sand islands near shipping routes between the Gulf of Mexico, Caribbean, and Atlantic. Spanish explorer Juan Ponce de León named the islands Las Tortugas in 1513 for the turtles his crew found; the later word Dry warned mariners that freshwater was scarce. After the United States acquired Florida, military planners recognized Garden Key's strategic position near the Florida Straits. A lighthouse appeared in the 1820s, and construction of Fort Jefferson began in 1846 as part of a national system of coastal defense.

Engineers planned a six-sided masonry fortress with two tiers of casemates, hundreds of guns, a moat, magazines, barracks, officers' quarters, and cisterns. More than sixteen million bricks were transported across the sea, and enslaved laborers, soldiers, prisoners, civilian craftsmen, and contract workers all contributed. The site's remoteness, hurricanes, heat, disease, shifting foundations, and difficult supply made construction extraordinarily demanding. New rifled artillery also threatened brick fortifications before completion. Fort Jefferson became immense but was never fully armed or finished according to its original design.

Union forces held the fort throughout the Civil War, protecting a naval base and denying the harbor to the Confederacy. It also served as a military prison. Four men convicted in the conspiracy surrounding Abraham Lincoln's assassination arrived in 1865, including physician Samuel Mudd, who had treated John Wilkes Booth's broken leg. During a yellow-fever outbreak in 1867, Mudd provided medical help after the post surgeon died; President Andrew Johnson pardoned him in 1869. His story remains contested because assistance during the epidemic did not erase the conviction or wider context.

The army abandoned Fort Jefferson in 1874, although federal agencies reused it for quarantine, naval, lighthouse, and scientific purposes. Hurricanes, salt, settlement, metal corrosion, and scavenging damaged interiors and gun platforms. President Franklin Roosevelt designated Fort Jefferson National Monument in 1935, and Congress created Dry Tortugas National Park in 1992 to protect the fort, islands, reefs, wildlife, and submerged resources. Conservation repoints brickwork, repairs the moat wall, manages corrosion, and stabilizes ruins. Lost barracks and fittings have not been comprehensively reconstructed, leaving open courtyards and partial interior remains.

The National Park Service manages the remote park, reached only by boat or seaplane. Ferry, seaplane, private-vessel, camping, and park-fee arrangements differ, but once on Garden Key visitors can follow routes through much of the fort subject to safety closures. The exterior walls, casemates, moat, parade ground, and many service spaces retain extensive nineteenth-century fabric, while docks, signs, railings, utilities, and campsites support modern use. Fort Jefferson's incompleteness is historically meaningful: it records the rapid obsolescence of masonry defense as clearly as it records wartime garrisoning, imprisonment, navigation, and preservation.