During the American Revolutionary War, the British Army bombarded and captured the fort as part of their conquest of Philadelphia in autumn 1777.

Built in 1681 in Philadelphia near the confluence of the Delaware and Schuylkill Rivers, Fort Mifflin was recognized as strategically important because of the role it played in defense of the settlement.

[5] Since the Quakers rejected the military, they instead sought to make peace with Native American tribes in the area and avoid any need to fortify their settlements militarily.

In response to complaints by the Secretary of State for the Colonies, Pennsylvania governor John Penn asked General Thomas Gage to send someone capable of designing defenses for the city.

[9][nb 2] The commissioners reviewed the plans, found them all too expensive, and insisted on economy despite Montresor's protestations about the budget.

"[10] The colonial Provincial Assembly passed a bill releasing £15,000 for the construction of the fort and the purchase of Mud Island from Joseph Galloway, the Speaker of the House.

On 4 June 1772, Montresor left the head workman in charge of the construction project and returned to New York disgruntled.

[13] Following the adoption of the Declaration of Independence, Benjamin Franklin headed a committee to provide for the defense of the revolutionary capital of Philadelphia.

[15] Defenders of Philadelphia assembled chevaux de frise obstacles,[14] placed in tiers spanning the width of the Delaware between Forts Mercer and Mifflin.

These defenses comprised wooden-framed "boxes", 30 feet square, constructed of huge timbers and lined with pine planks.

They placed two or three large timbers tipped with iron spikes into each frame, set underwater and facing obliquely downstream.

Similar obstacles were built downriver at Fort Billingsport, New Jersey, but that area fell to the British on October 2, 1777.

[14] During the siege, four hundred American soldiers were besieged by two thousand British troops and a Royal Navy squadron.

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 their river operations while patrolling the waters around the fort.

Their resistance had effectively denied the Royal Navy free use of the Delaware River and allowed the successful repositioning of the Continental Army for the Battle of White Marsh and subsequent withdrawal to Valley Forge.

The ruins of Fort Mifflin lay derelict until 1793, when rebuilding began under what was later called the first system of U.S. coastal fortifications.

In 1794, Pierre L'Enfant, also responsible for planning Washington, D.C., supervised the reconstruction, including the design and rebuilding of the fort.

[24] 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.

The Commandant's House exemplifies Greek Revival architecture, rare on Army installations in the United States.

The army built the blacksmith shop before 1802; it is probably the oldest surviving complete structure at Fort Mifflin.

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.

In 1837, the hospital and mess hall building was converted to a meetinghouse[28] and an artillery shed, for the storage and protection of cannon, was built on an interior raised platform.

The Union Army accused William Howe, one of its soldiers, of desertion, found him guilty of murder, and imprisoned him famously at Fort Mifflin from January 1864.

Despite his illiterate reputation, Howe twice wrote letters (filled with bad grammar and run-on sentences) to President Abraham Lincoln asking for clemency, signing them with his own hand.

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 two hundred.

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 with A Company, 7th United States Veteran Volunteers, relieved Lieutenant Colonel Eastman.

In the late 1970s, the Commandant's House at the Fort was destroyed by an accidental fire started by camping Boy Scouts.

In August 2006, 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.