Fort Mifflin was first called Fort Island Battery on the Delaware River below Philadelphia, Pennsylvania. Also known as Mud Island Fort, it was commissioned in 1771.

By the 1740s, the site ranked as the wealthiest British port in the New World. French and Spanish privateers entered the Delaware River then, threatening the city. During King George’s War between 1744 and 1748, Benjamin Franklin raised a militia because the city’s legislators, most of whom were Quakers, opposed military engagement and refused to defend Philadelphia “either by erecting fortifications or building Ships of War.” Franklin raised money to create earthwork defenses and to buy artillery. At the war’s end, commanders disbanded the militia and left the city’s defenses derelict.

With renewed colonial warfare in the 1750s, especially the French and Indian War, plans were drawn up for a fort on Mud Island, but no fort was built.

After General George Washington’s defeat at the Battle of Brandywine, the British took control of Philadelphia in September 1777. The British forces then laid siege to Fort Mifflin and Fort Mercer in early October 1777, unsuccessfully attacking the latter by land and river in the Battle of Red Bank on October 22. The British intended the siege to open its supply line via the Delaware River. Captain John Montresor, earlier designer and constructor of Fort Mifflin, planned and built the siege works against Fort Mifflin. He then led the siege and destroyed much of the fort. During the siege, 400 American soldiers held off more than 2,000 British troops and 250 ships until November 10, 1777, when the British intensified their assault, launching a constant barrage of cannonballs into the fort. Among those stationed at the fort was Private Joseph Plumb Martin, who later wrote an account of the battle.

Defending the riverway, Commodore John Hazelwood, with a sizable fleet of galleys, sloops, and fire vessels, launched several raids on British positions on shore, constantly harassing British river operations while patrolling the waters around the fort. On November 15, 1777, the American troops evacuated the fort. Their stand effectively denied the British Navy free use of the Delaware River. It allowed the successful repositioning of the Continental Army for the Battle of White Marsh and subsequent withdrawal to Valley Forge. Fort Mifflin experienced the heaviest bombardment of the American Revolution. The siege left 250 of the more than 400 men garrisoned at Fort Mifflin killed or wounded. Comrades-in-arms ferried these dead and wounded to the mainland before the final evacuation. Fort Mifflin never again saw military action.

Over a cross-shaped hole in the ground previously designated as a last-ditch defensive area near the center of the fort, the army built the extant citadel structure to house the commandant in 1796.

Lieutenant Colonel Stephen Rochefontaine replaced Pierre Charles L’Enfant as chief engineer at Fort Mifflin in 1798 and completed the citadel structure to house the commandant. Lieutenant Colonel Rochfontaine used and improved the original designs of L’Enfant. The Commandant’s House exemplifies Greek Revival architecture, which is rare on Army installations in the United States. The army also built the six cavelike casemates as defensive structures in the case of an enemy siege during the reconstruction of 1798-1801. Soldiers used a “bake oven” just inside the main gate and the entrance to the bomb-proof casemate for baking bread, as a chapel, and as a mess hall.

In 1814, a two-story officers’ quarters measuring 96 feet by 28 feet was built.

The U.S. Army built a one-story brick structure, 24 feet by 44 feet, in 1815-1816 as a guardhouse and prison. In about 1819, north of the fort’s walls, the army also built a two-story building that housed a hospital on the second floor and a mess hall on the ground floor.

Fort Mifflin was relegated to secondary status after the construction of Fort Delaware in 1820. During the 19th century, the area around the fort was drained and filled until Mud Island connected with the western bank of the Delaware River. Nevertheless, the building and garrisoning of Fort Mifflin continued. In the early 1820s, the army began meteorological observations at the fort.

During the Civil War, the Union used Fort Mifflin to house Confederate prisoners of war, as well as Union soldiers and civilian prisoners. Numerous Confederate prisoners occupied Fort Mifflin from 1863 to 1865 and were housed in Casemate #1. The Union Army used three smaller casemates to hold political prisoners during the same period. During that time, the fort had a maximum prisoner capacity of only 250, but the most it housed at one time was 215. There were 42 escapes and three prisoner deaths recorded.

The Union Army accused William H. Howe, one of its soldiers, of desertion, found him guilty of murder, and imprisoned him famously at Fort Mifflin from January 1864. Howe led an attempted escape of 200 prisoners from Casemate #5 in February 1864. Afterward, Howe was housed in a solitary confinement cell in Casemate #11, where he left his signature. Despite his illiterate reputation, Howe twice wrote letters (filled with bad grammar and run-on sentences) to President Abraham Lincoln, asking for clemency and signing them with his own hand. In April 1864, Howe was transferred to Eastern State Penitentiary but was transferred back to Fort Mifflin on August 26 of the same year. The condemned prisoner was briefly held in the fort’s wooden guardhouse before his execution on the gallows, which were steps away from the guardhouse. Howe’s hanging was before an audience of people who paid for tickets to watch the execution. Of the three other men executed at Fort Mifflin, none had a paid public audience.

On November 24, 1864, the Union Army sent Lieutenant Colonel Seth Eastman, the American Western frontier painter, to Fort Mifflin to supervise the discharge of all civilian and military prisoners, then numbering more than 200. On January 2, 1865, Eastman reported that his garrison consisted of B Company, 186th Pennsylvania Volunteer Regiment, a detachment of recruits, and the hospital staff.

On August 20, 1865, Captain Thomas E. Merritt relieved Lieutenant Colonel Eastman with A Company of the 7th United States Veteran Volunteers. The army completed the West sally port by 1866, and the same year, the 7th United States Veteran Volunteers vacated the fort. The District Engineer Office, Corps of Engineers, replaced the company. The fort passed in and out of use several times in its subsequent history.

Between 1866 and 1876, the Corps of Engineers intermittently repaired and modernized Fort Mifflin and upgraded its armament. The army worked on the detached high battery south of the fort from 1870 to 1875 but never finished it. The army also built a torpedo casemate in 1874-1875. Today, Casemate #11″, preserves a trove of historical artifacts. These artifacts include pottery, a tin cup, a tin chamber pot, period buttons, and dozens of animal bones.

From 1876 to 1884, the Philadelphia District Office of the Corps of Engineers took custodial responsibility for Fort Mifflin. The east magazine (torpedo casemate) first appeared on a map in 1886. During World War I, the fort was used to store munitions.

The old fort was disarmed and partially dismantled in 1904.

The fort was designated a National Historic Landmark in 1915. The army removed the two-story kitchen wings from the officers’ quarters building sometime before the 1920s.

The fort was restored from the original plans in 1930.

During World War II, the U.S. Army stationed anti-aircraft guns at Fort Mifflin to defend the nearby Fort Mifflin Naval Ammunition Storage Depot and the Philadelphia Naval Shipyard. Marine Corps units from the naval shipyard guarded the Naval Ammunition Storage Depot at the northern end of the former Mud and Cabin Islands, and the U.S. Army assigned troops to defend the fort. In April 1942, the U.S. Army stationed Battery H of the 76th Coast Artillery Regiment, the first African American Coast Artillery unit in U.S. military history, at the fort. On May 24, 1942, the 76th Regiment was relieved and moved to California to prepare for overseas deployment, and the U.S. Army stationed the 601st Coast Artillery Regiment at Fort Mifflin.

In 2006, Wayne Irby rediscovered and unearthed the recently named Casemate #11 at Fort Mifflin. In August of that year, Dr. Don Johnson and a small group of volunteers uncovered and rediscovered the complexity of the fort’s inner rooms and a trove of historical artifacts inside Casemate #11.

Fort Mifflin remains an active base for the U.S. Army Corps of Engineers. It is currently the oldest active U.S. military base and the only base predating the Declaration of Independence.

Only the white stone walls of the original Fort Mifflin survive today. The pockmarks in these stone walls evidence the intensity of the British bombardment of 1777. The standing buildings that remain today are the Arsenal, Artillery Shed, Blacksmith Shop, Sutler Building/Storehouse, Soldiers’ Barracks, Officers’ Quarters, Commandant’s House, Hospital/Messhall, West Sallyport, Casemates, East Magazine, and Casemate #11. Today, the site is a National Historic Landmark and offers grounds tours.