Joseph Hopkins enlisted in the New York National Guard in 1916, served with the 106th Machine Gun Battalion in World War II, and then was discharged to pursue studies at Columbia University. He was appointed a flying cadet in 1925 and commissioned in the Air Reserve, receiving his regular commission in the Air Corps in 1927. After serving in the Army Air Forces in World War II, he transferred to the U.S. Air Force when it became a separate branch of service in 1947. He retired in 1952 as a U.S. Air Force Brigadier General.