In November 1940, Kerr entered active duty as inspector general of the 36th Division, and he later served as executive officer of the 72nd Infantry Brigade before becoming the division's chief of staff in 1942. He served in this position until 1944, and took part in the North African and Italian campaigns. In February 1944, he was assigned as deputy commander of the Allied Military Mission to the Italian Army. In this role, he worked to integrate Italian forces into the Allied war effort following the overthrow of Benito Mussolini's dictatorship, including participation in the 1945 offensive which brought about the surrender of Nazi forces in Italy. After the war, Kerr negotiated with the Italian government for land on which to place a monument to commemorate the 36th Division's 1943 landing at Salerno. Kerr was a frequent contributor of articles to professional journals and other publications, including 1944's "Joint Maintenance in Theater Operations". In addition, he authored a 1945 article on the operations of U.S. forces in the Pacific Ocean and South West Pacific theaters, "The Organization of the Pacific Ocean Areas and of the Southwest Pacific Area".