While at college, Greg completed the Civilian Pilot Training Program and was subsequently accepted into the Army's pilot training program. Earning his wings in 1942, Greg became a P-38 combat pilot and served in North Africa during the summer of 1943-a critical time when the Luftwaffe was still a potent threat, and America had begun the march northward from the Mediterranean into Europe proper.
Following the war, Greg served with a B-29 unit, then transitioned to the new, red-hot B-47 strategic bomber. In his frequent deployments, he was always assigned the same target in the Soviet Union-Tbilisi, Stalin's home town. While a B-47 pilot, Greg was selected to join America's first high-altitude program-the Black Knights. Flying RB-57D aircraft, Greg and his team flew peripheral "ferret" missions around the Soviet Union and its satellites, collecting critical order-of-battle data so desperately needed by the Air Force at that time. When that program neared its design end, and following the Gary Powers shoot-down over the Soviet Union, Greg was assigned to command of the CIA's U-2 unit at Edwards AFB. It was during that five-year command that Greg and his team provided critical overflight intelligence, including during the Bay of Pigs, the Cuban Missile Crisis, and the Vietnam build-up. He found time to also become one of the first to fly U-2s off aircraft carriers in a demonstration project.
Following his U-2 command, Greg attended the National War College, was assigned to the reconnaissance office at the Pentagon, and then was named Vice-Commandant of the Air Force Institute of Technology (AFIT). Greg retired from the Air Force in 1972.