The first American to down five enemy aircraft during World War I, Libby never flew a combat mission for the United States Air Service. He became an ace while serving as an observer in the Royal Flying Corps. When the war began, Libby was in Canada where he joined the Canadian Army. In 1916, he was serving in an ambulance unit in the trenches of France when he volunteered to join the Royal Flying Corps.

Later that year, as an F.E.2b observer, he became the first American ace of the war. In 1917 Libby completed pilot training and was assigned to 43 Squadron on April 18. After scoring 2 victories, he was reassigned to 25 Squadron as a D.H.4 pilot in August of 1917. At the request of General Billy Mitchell, Libby transferred to the American Air Service on September 15, 1917. Returning to the United States, he participated in the Liberty Loan drive before joining the 22nd Aero at Hicks Field in Texas. Unfortunately, Libby was seriously ill by this time and was found to be permanently disabled and medically unfit for military service.

Military Cross (MC)
"For conspicuous gallantry in action. As an observer, he, with his pilot, attacked four hostile machines and shot down one. He has previously shot down four enemy machines." MC citation, November 14, 1916