Leahy was initially assigned to the 38th Infantry at Fort Sam Houston, Texas. In 1942, he was transferred to the 82nd Airborne Division at Fort Bragg, North Carolina. Leahy was promoted to major in January 1943 and then served with the division in North Africa. He participated in the invasion of Sicily and then in the landings at Salerno, Italy.[1][6]
In June 1944, Leahy participated in the invasion of Normandy and temporarily assumed command of the 2nd Battalion, 325th Glider Infantry after his commanding officer was gravely wounded. A few days later, he assumed command of the 3rd Battalion, 325th Glider Infantry after the unit suffered significant casualties and lost three commanding officers in five days. Leahy reorganized the unit into an effective fighting force and continued to lead it until May 1945. He participated in Operation Market Garden, dropping into Nijmegen in September 1944. Leahy was promoted to lieutenant colonel on 16 December 1944, and then participated in the Allied counterattack during the Battle of the Bulge. He earned three Silver Stars for his bravery and leadership in combat.[1][4]
After the war, Leahy served as an instructor at the Infantry School at Fort Benning before returning to Fort Bragg. While conducting a test jump with a new type of parachute, it opened too rapidly, breaking both of his collarbones and catching on the airplane tail section. Leahy had to cut himself loose with his knife and land using his reserve chute. From 1953 to 1955, he served with the Military Assistance Group in the Philippines. Leahy was promoted to colonel in July 1955.[2][4][5]
After attending the Army War College, Leahy served as commander of the 505th Airborne Infantry Regiment at Fort Bragg. From 1961 to 1962, he served with the Military Assistance Group in South Korea. From 1962 to 1964, Leahy taught at Air University in Alabama.[3][4][5]
In September 1964, Leahy was promoted to brigadier general and then served as chief of the Military Training Mission in Saudi Arabia until August 1966. From 1966 to 1967, he served as chief of staff of XVIII Airborne Corps at Fort Bragg.[3][5][7]
Promoted to major general in 1967, Leahy served as commanding general of II Corps at Fort Wadsworth, New York. In 1968, he served as commanding general of the 7th Infantry Division in South Korea. Leahy then became head of the Institute of Land Combat at Fort Belvoir, Virginia.[3][4]
From 1970 to 1972, Leahy was chief of staff and deputy commanding general of the 6th Army at the Presidio of San Francisco. He retired in 1972, but was immediately recalled to active duty as head of the Military Disability Agency at the Walter Reed Medical Center in Washington, D.C. Leahy retired again in 1977.[4][5][6]
During his career, Leahy received the Distinguished Service Medal, two awards of the Legion of Merit and a Bronze Star Medal.[4][6][7]