Millis was a staff member of the Fund for the Republic from 1954 to 1968.
Millis, widely recognized as a historical writer, wrote eight books including: Road to War: America 1914–1917, This is Pearl!
The United States and Japan—1941, Why Europe Fights, Viewed Without Alarm: Europe Today, Arms and Men: A Study of American Military History, The Martial Spirit: A Study of Our War with Spain, and An End to Arms.
He graduated from Yale University, although his studies were interrupted by World War I, when he joined the Army and became a second lieutenant in the field artillery.
His second marriage, to fashion journalist Eugenia Sheppard in 1944, ended with his death.