In January 1943, after graduating from the United States Military Academy, Knowlton was commissioned in the cavalry. He reported for duty with the 7th Armored Division, and during World War II led an assault gun platoon in France, and later a reconnaissance troop in Germany, which linked up with the Russians, advancing from the east, in Ganzlin, northwest of Berlin. For this he was awarded the Silver Star.
After his time in Vietnam, he became Secretary of the Army General Staff, and on 23 March 1970 he became the 49th Superintendent of the United States Military Academy, an assignment he held for four years. In 1974, during his tenure, the United States Supreme Court reaffirmed West Point's right to enforce the Honor Code in response to two challenges from cadets.
After his time as Superintendent, he was assigned as Chief of Staff of the United States European Command. Promoted to full General in 1976, he took command of Allied Land Forces South East Europe, and finished his career as the United States Representative on NATO's Military Committee.