Fort Mifflin was originally built by the British in 1771. But during the American Revolutionary War, the Colonists used it along with Forts Mercer and Billingsport to keep the British supply lines at bay while General Washington marched his defeated troops to Valley Forge. When troops arrived at Valley Forge, it was there that Washington turned the tide of the war.

Even up to the Civil War, Fort Mifflin was used as a prison camp. It remained the oldest fort in constant use in American history from 1771-1952.