During World War I, Fort Mifflin was not garrisoned by a large cache of troops, nor did it see combat. Nevertheless, the Fort played a vital role in serving Philadelphia's home front efforts during the conflict.
Fort Mifflin was commissioned in 1771 as a coastal fort to protect Philadelphia from attack or invasion. During the American Revolutionary War, it was the site of a six-week siege by the British (October-November 1777) that culminated in a six-day bombardment (November 10-15, 1777) that some called the heaviest of the war. In the 1790s, the young federal government rebuilt Fort Mifflin as part of the First American System of Fortification, a network of coastal forts designed to protect America's ports. During the Civil War, the Fort served as a Union Army prison.
Prior to World War I, Fort Mifflin was in an utter state of disrepair. The small, outdated Fort had not held a garrison since April 1866 nor held artillery since 1904. At the time, Fort Mifflin primarily served as a tourist destination, especially popular among veterans groups and fraternal organizations. In 1915 the War Department declared Fort Mifflin a national historic monument. 
However, the fort's role changed following the outbreak of World War I. In order to prepare Philadelphia for the conflict, “local war planners developed a scheme for outer and inner lines of entrenched positions about Philadelphia, that required for the strengthening of Forts Du Pont, Mott, Delaware, and the restoration of Fort Mifflin.” To that end, the War Department provided money to modernize some of Fort Mifflin's crumbling structures. In April 1917 District Engineer Major Mark Brooke allocated $8,000 for Fort Mifflin to repair the barracks, hospital, commandant’s house, and one magazine.  Repairs concluded in August 1917, preparing Fort Mifflin for military service once again. 
With Fort Mifflin restored, the important question remained of what role the small 18th century coastal fort would serve. The initial intention was to use Fort Mifflin as a storage yard to house locomotive engines and other machinery to be sent to Europe. However, the Fort's distance from Philadelphia's industrial center made it unfit for this task. Instead, beginning in 1918 the Fort served as part of the Fort Mifflin Naval Ammunition Depot. The Depot was a large facility, further expanded during the war to include “three large storage sheds, four brick magazines, and other buildings.”  Additionally, naval engineers laid out a small railroad, connecting Fort Mifflin to the adjacent Army Corps of Engineers Depot and the Naval Ammunition Depot.
After the war ended, the Naval Depot continued to store millions of pounds of ammunition. However, the storage of ammunition so close to Philadelphia prompted public anxiety (one local newspaper called the Depot “an ever-present menace to the city”). As a result in 1929, the Navy redistributed the munitions, and Fort Mifflin once again entered a state of inaction.  The United States Army officially decommissioned Fort Mifflin in 1954.
 Preservation and repair, fortifications. 1 July 1915; Brooke to Chief of Engineers, 19 April, 1 June, 1 August, 1 September 1917, allin Press Copy Book: Reports: Philadelphia Office, Delaware City Office, Wilmington Office, 1911-1917, Records of the Philadelphia District U.S. Army Corps of Engineers, RG 77, Mid-Atlantic Regional Branch, National Archives Philadelphia.
 Parsons to J. Henry Miller, 14 and 19 May 1918, Records of the Philadelphia Navy Yard, box 13, Records of the 4th Naval District, Records of the Naval Districts and Shore Establishments, Record Group 181, Mid-Atlantic Regional Branch, National Archives, Philadelphia.