Appointed to the United States Military Academy (West Point) in 1917, his class graduated from an accelerated academic program in 1918 with the anticipation of entering the First World War.

[4] Hartness attended Columbia College in New York City in anticipation of serving on the West Point faculty, and he received a Bachelor of Arts degree in English in 1923.

[2] After returning to the United States, he served as assistant professor of military science for the Army Reserve Officers' Training Corps program at the University of Minnesota.

[2] From 1933 to 1935, Hartness was a student at the United States Army Command and General Staff College, and he was promoted to captain in June 1935.

[2] In Hartness' view, officers in the German army were naive with respect to Hitler's plans and disconnected from public opinion.

[2] From 1937 to 1940, Hartness served at the Command and General Staff College as an instructor in European tactics and strategy, and he was promoted to major in July 1940.

[2] Hartness served with the 26th Division until the end of the war, including combat in the Northern France, Rhineland, Ardennes Alsace, and Central Europe campaigns.

[2] When the National War College was founded in 1946, Hartness was among the senior officers whose wartime experience resulted in them being awarded equivalent credit for completing its program of instruction.

[2][8] In April 1951, Hartness led the 4th Infantry Division as it moved to West Germany in order to take up its role as part of North Atlantic Treaty Organization (NATO) forces in Western Europe, the first large deployment of U.S. troops to the European continent since the end of World War II.

[2] Sarah Elizabeth Hartness was the wife of Allan Rawson Williams, an army veteran of World War II and a research chemist for Uniroyal.