In 1775, Commodore Esek Hopkins ordered his vessels to fly a striped flag, which became the first Naval Jack Flag. Now known as the Fort Mifflin Flag, it has a very specific design. 13 alternating stripes of red, white, and blue to symbolize the 13 original colonies. The original flag was three to four times bigger than the flag that currently flies over the fort today. The flagpole was also three to four times bigger than its modern-day counterpart because originally, the soldiers used a mast from a big, tall naval ship. During the American Revolution, the Fort Mifflin Flag could be seen from the city of Philadelphia.

Private Joseph Plumb Martin, who was stationed here during the Siege of Fort Mifflin, observed in his writings some interesting facts about the flags that have flown here. Needing to raise a flag of distress, officers asked who wanted to do it. Just before Martin could speak up, a sergeant of the artillery volunteered. He quickly came back down so as not to be struck by enemy fire. He had not gotten three yards away from the mast before Martin wrote: “he was cut in two by a cannon shot”.