Kerwin graduated from the United States Military Academy in 1939 and was commissioned in the field artillery and assigned to the 3rd Infantry Division.

In December 1944, while in France, he was wounded in Mutzig and evacuated to the United States, returning to Europe in 1945 and assigned to the Theater Operations Division of the War Department's General Staff.

As a brigadier general Kerwin took command of the 3rd Armored Division Artillery in Hanau, Germany in August 1961.

Recognized by his commanders and peers as an innovative artilleryman, Kerwin developed a system of massing fires that contributed immeasurably to the success of the Allied landings at Anzio Beachhead.

Having been a member of the board of directors of the Army and Air Force Mutual Aid Association since 1969, he became its chairman in 1982, serving in that capacity until 1997.