The Weston, Massachusetts, native began his career as a Second Lieutenant in the Armored Cavalry and fought in four campaigns during World War II, beginning in Normandy. In the last weeks of the war, he was awarded a Silver Star for leading a reconnaissance mission deep behind German lines to make one of the first contacts with the Soviet forces north of Berlin.

His later commands battalion and brigade armored cavalry and armor units, the 9th Infantry Division's multi-brigade force in South Vietnam and the Allied Land Forces Southeastern Europe in Izmir, Turkey.

He also served on the staffs of General Dwight D. Eisenhower and General Omar Bradley at the Supreme Headquarters Allied Powers Europe of the North Atlantic Treaty Organization in the early 1950s.

He was on the staff of the Military Assistance Command, Vietnam from 1966 to 1968, where he oversaw civil operations on General William Westmoreland's staff and served as Assistant Division Commander in the 9th Infantry Division. His work in Southeast Asia resulted in the award of a Distinguished Service Medal, Distinguished Flying Cross, Bronze Star and 10 Air Medals. He also received two more Silver Stars, one for gallantry at a fire support base that came under sudden attack and the other in a battle on the Plain of Reeds.

He was on the general staff of the secretary of Army for the next two years until he went to West Point, where his daughter Hollister met and married Lieutenant David Petraeus, now a four-star General and Commander of the Multinational Forces in Iraq.

A soldier-scholar, General Knowlton also taught social sciences at West Point while working on a master's degree in political science at Columbia University, which he received in 1957. He also graduated from the U.S. Army Command and General Staff College and the National War College.

After West Point, General Knowlton became Chief of Staff of the European Command and for his last three years of active duty was the U.S. representative to NATO's military committee in Brussels, the highest military authority in the NATO alliance.

After his retirement, he was a senior fellow at a defense studies institute at the National Defense University at Fort McNair for 15 years. He also served as an adviser for the Defense Nuclear Agency and was a member of the Defense Intelligence Agency Science and Technology Advisory Board.