Letterkenny Army Depot (originally Letterkenny Ordnance Depot), the Center of Industrial and Technical Excellence (CITE) for Air Defense and Tactical Missile Systems, was established in early 1942. Its leadership began recruiting civilian personnel in July 1942.

The Depot is under the command structure of the U.S. Army Aviation and Missile Command (AMCOM). The facilities at Letterkenny are used to conduct maintenance, modification, storage, and demilitarization operations of tactical missiles and ammunition.

Located primarily in Letterkenny Township and extending into Greene Township and Hamilton Township, all in Franklin County, Pennsylvania, just northwest of the borough of Chambersburg, the Depot consists of nearly 18,000 acres (71 km²). It is the largest employer in Franklin County, Pennsylvania, and adds over one-quarter of a billion dollars annually to the region's economy.

Letterkenny has unique tactical missile repair capabilities repairing a variety of Department of Defense (DoD) missile systems, including the MIM-104 PATRIOT missile and its ground support and radar equipment. More recently, Letterkenny expanded its product line to include designation of the CITE for Power Generation for the Army, the overhaul of tactical wheeled vehicles (HMMWVs), material handling equipment (7.5-ton cranes), and Mobile Kitchen Trailers. In 2007, during the Iraq conflict, Letterkenny began building new Mine Resistant Ambush Protected (MRAP) armored vehicles in partnership with BAE Systems, and in 2010 was designated the Joint Depot Source of Repair (JDSOR) for Route Clearance Vehicles for the DoD.

In 1941, Letterkenny was chosen by the U.S. Department of War to be one of twelve new ordnance depots. On December 18, 1941, the Secretary of War, Henry L. Stimson, issued the directive to acquire 21,000 acres (85 km²) at Letterkenny for a depot. The mission would be to reduce the surplus of forthcoming material and to store and ship ammunition, trucks, parts, and other supplies. Construction on the depot took place from January through April 1942.

In addition, the government obtained the James Finley House in 1942 for use as the Commanding Officer's Residence, making it one of the largest depots of its kind.

Depot military officers then began recruiting civilian personnel in July 1942.

The first shipment of ammunition arrived by train in late 1942, and more than three million tons of supplies were moved during World War II. Beginning in 1943, news was disseminated internally to civilian and military employees via a civilian-sponsored, weekly newsletter, the "Kenny Letter." In August 1945, newsletter editors marked the second anniversary of their publication and the war's end by producing a special "V-J edition" (Victory over Japan Day) with the word "Victory" printed in large, red lettering as the front-page headline. The depot also sponsored the Letterkenny Ordnance Depot Baseball Team (L.O.D.), a civilian group which participated in the Twilight Baseball League with teams from the Chambersburg Engineering Company (CECO) and the Pennsylvania Railroad (P.R.R.) in night games at Henninger Field in Chambersburg during the summers of 1943 and 1944. Joining the civilian team from the depot for the 1944 season were the CECO and P.R.R. teams, as well as new teams from St. Thomas High School and the U.S. Army's 831st Ordnance Company. The five teams participated in a forty-game schedule that summer. The P.R.R. Team defeated Letterkenny's civilian team 2–1 in the seventh game of the league's 1944 championship playoff on August 25.