Major John J. Duffy was commissioned an Infantry Officer upon completion of Officer Candidate School Class 4A-63.

His subsequent duty assignments include: Company Commander, HHC, 2nd Battalion, 6th Infantry, Berlin Brigade, Berlin, Germany; Commanding Officer, Detachment A-101 (Lang Vei Special Forces Camp), and Executive Officer, Detachment A-113 (Mobile Strike Force), Company C, 5th Special Forces Group, 1st Special Forces, Republic of Vietnam; S-2, Detachment B-55, HHC, 5th Special Forces Group, 1st Special Forces, Republic of Vietnam; Commanding Officer, 48th Company, 4th Student Battalion, The Student Brigade, Fort Benning, Georgia; Operations Officer, Studies and Observation Group, Military Assistance Command, Republic of Vietnam; Liaison Officer and Assistant Launch Officer with the 7th Air Force, Studies and Observation Group (SOG), 5th Special Forces Group, 1st Special Forces, Republic of Vietnam; Training Officer, HHC, 5th U.S. Army, Fort Sheridan, Illinois; Chief, Operations section, SOG, Military Assistance Command, Republic of Vietnam. Commander, Battalion Combat Assistance Team, Airborne Advisory Detachment (Team 162), Military Assistance Command, Republic of Vietnam; Advisor, Company A, 11th Special Forces Group, Fort Devens, Massachusetts; Operations Officer, U.S. Army Support, Thailand; Operations Officer and Deputy Commander, Joint Casualty Resolution Center, Thailand; Directory, Plan Training and Security, Commander of Troops and Chief, Foreign Liaison and Protocol, Fort Hamilton, New York.

His military awards and decorations include: Medal of Honor, Bronze Star Medal (V device 4 Oak Leaf Cluster), Air Medal ( V device, Numeral 6), Army Commendation Medal ( V device), Soldiers Medal, Purple Heart (8 Oak Leaf Cluster), Presidential Unit Citation (Army), Presidential Unit Citation (Navy), Valorous Unit Award, Meritorious Service Medal, Joint Service Commendation Medal, Meritorious Unit Citation, Vietnam Service Medal, American Occupation Medal (Berlin), Armed Forces Expeditionary Medal, National Defense Service Medal, Good Conduct Medal, Armed Forces Achievement Medal (Republic of Vietnam), Vietnam Cross of Gallantry (Silver Star, 2 awards), Vietnam Cross of Gallantry (with Palm, 2 awards) and the Armed Forces Reserve Medal.