John J. Gallagher enlisted in the U.S. Navy in September 1968. He completed Boot Camp and advanced electronics training at the Naval Training Center in Great Lakes, Illinois. Upon successfully completing his training in December 1969, Gallagher received orders to Keesler AFB in Biloxi, Mississippi, for additional electronics training. He was then assigned to the Naval Amphibious Base in Coronado, California, where he completed SERE and small arms training.

In April 1970, Gallagher traveled to McChord AFB in Tacoma, Washington, for his flight to Cam Ranh Bay, Republic of Vietnam. Upon arrival, he was transported to the U.S. Naval Communication Station RVN, and within three days, he was sent to a remote transmitter site. His primary duty was to maintain and repair communication equipment.

In April 1971, Gallagher returned stateside and was assigned to the Naval Submarine Base New London in Groton, Connecticut. In January 1974, he was assigned to the USS Orion (AS-18), a submarine tender homeported in North Charleston, South Carolina. Gallagher was honorably discharged in September 1974, having achieved the rank of Electronics Technician 2nd Class (E-5). His awards include the National Defense Service Medal, Vietnam Service Medal with one star, Vietnam Campaign Medal with 60 Device, Navy Good Conduct Medal, and the Navy Unit Commendation.

In November 1979, John J. Gallagher joined VFW Post 4927. He has since held several leadership positions, including Surgeon, Chaplain, Senior Vice Commander, Commander, and Adjutant.