[2] As director of the United States Aeromedical Research Laboratory, he applied his medical and aviation knowledge to the improvement of aircrew protection from temperature extremes and the lack of oxygen at high altitude.

He attended the University of Minnesota, but left after one year to enlist in the US Marine Corps, serving with them as a private from October 1918 to March 1919.

Serving alternately in England and the United States, in 1945 he became surgeon for the Air Division in the office of Military Government for Germany (U.S.), with headquarters in Berlin.

In 1954, General Armstrong went to Wiesbaden, Germany, as Surgeon of the United States Air Forces in Europe.

In 1977, the Air Force Aerospace Medical Research Laboratory created the Harry G. Armstrong Award for Scientific Excellence.