In late 1939, Borella changed his surname by deed poll to "Chalmers-Borella" in honour of his late mother, who had died when he was four years old. Borella enlisted in the Second World War, and served in Australia from October 1939 to May 1945, obtaining the rank of captain. He initially served in the 12th Australian Garrison Battalion until July 1941 when he was posted to the Prisoner of War Group, based at Rushworth, Victoria, before being posted to the 51st Garrison Company, based at Myrtleford, Victoria.