Jack Koelsch was born on December 22, 1923, in London, England. He attended Princeton University in Princeton, New Jersey, for a year before enlisting in the U.S. Navy Reserve on September 14, 1942. Koelsch went on active duty as a Naval Aviation Cadet on January 4, 1943, and he was designated a Naval Aviator and commissioned an Ensign in the U.S. Navy at NAS Pensacola, Florida, on October 24, 1944. He then attended operational training at NAS Fort Lauderdale, Florida, from October 1944 to February 1945, followed by service at Headquarters, Air Force, Atlantic Fleet, at NAS Norfolk, Virginia, from February to March 1945. Ensign Koelsch attended TBM Avenger replacement pilot training with Composite Squadron 15 (VC-15) at NAS Norfolk from March to June 1945, and then served as a TBM pilot with Torpedo Squadron 97 (VT-97) from June to December 1945. His next assignment was as a TBM pilot with VT-18 at NAS Quonset Point, Rhode Island, from December 1945 to November 1946, followed by service with Attack Squadron 8A (VA-8A) from November 1946 to May 1948. LtJg Koelsch received an assignment to complete his bachelor's degree at Princeton University from May 1948 to February 1949, and he then served as Assistant Projects Officer with the Naval Aircraft Torpedo Unit at NAS Quonset Point from February 1949 to August 1950. He then became a helicopter pilot and served as Assistant Operations Officer and Officer in Charge of Helicopter Squadron One Detachment Eight with Helicopter Utility Squadron ONE (HU-1) at NAS Miramar, California, from August to October 1950. His next assignment was as Officer in Charge of the Helicopter Detachment aboard the aircraft carrier USS Princeton (CV-37) from October 1950 until he was forced down over enemy territory and listed as Missing in Action on July 3, 1951. LtJg Koelsch managed to evade capture for 9 before being taken as a Prisoner of War on July 12, 1951. He died while in captivity in North Korea on October 16, 1951, and his remains were returned to the United States after the war ended. LtJg Koelsch was buried at Arlington National Cemetery. The destroyer escort (later frigate) USS Koelsch (DE-1049/FF-1049) was named in his honor and served with the U.S. Navy from 1968 to 1989.

His Medal of Honor Citation reads:

For conspicuous gallantry and intrepidity at the risk of his life above and beyond the call of duty while serving with a Navy helicopter rescue unit. Although darkness was rapidly approaching when information was received that a marine aviator had been shot down and was trapped by the enemy in mountainous terrain deep in hostile territory, Lt. (j.g.) Koelsch voluntarily flew a helicopter to the reported position of the downed airman in an attempt to effect a rescue. With an almost solid overcast concealing everything below the mountain peaks, he descended in his unarmed and vulnerable aircraft without the accompanying fighter escort to an extremely low altitude beneath the cloud level and began a systematic search. Despite the increasingly intense enemy fire, which struck his helicopter on one occasion, he persisted in his mission until he succeeded in locating the downed pilot, who was suffering from serious burns on the arms and legs. While the victim was being hoisted into the aircraft, it was struck again by an accurate burst of hostile fire and crashed on the side of the mountain. Quickly extricating his crewmen and the aviator from the wreckage, Lt. (j.g.) Koelsch led them from the vicinity in an effort to escape from hostile troops, evading the enemy forces for nine days and rendering such medical attention as possible to his severely burned companion until all were captured. Up to the time of his death while still a captive of the enemy, Lt. (j.g.) Koelsch steadfastly refused to aid his captors in any manner and served to inspire his fellow prisoners by his fortitude and consideration for others. His great personal valor and heroic spirit of self-sacrifice throughout sustain and enhance the finest traditions of the U.S. Naval Service.