Chamberlin earned a bachelor of science at Oregon Agricultural College (now Oregon State University) in 1915, and graduated from the School of Military Aeronautics at University of California, Berkeley, on August 1, 1917. He passed Reserve Military Aviator's test on October 20, 1917 and was commissioned First Lieutenant in January 1918, with 141st Aero Squadron. During World War I he served in England, France, and Italy. He twice flew the English Channel delivering planes from Paris to London. He performed reconnaissance and photography work during the St. Miheil and Argonne offensives. In the second day of the St. Miheil drive with his observer, under great difficulties, carried out the longest reconnaissance over German territory of any American aviators. As a volunteer for a mission for flying at low altitude over enemy lines for certain information, he received the Cross of War from the French. He brought down three enemy planes and attempted to deliver messages to the famous Lost Battalion in the Meuse-Argonne Offensive, France. He was recommended three different times for the Distinguished Service Cross. He was discharged from active duty in February 1919 and became a professor at Oregon State.