Initially serving in the ranks as a private, Borella served with the 26th Battalion at Gallipoli from 12 September 1915 until being evacuated with jaundice on 19 November. He did not rejoin his unit until 5 February 1916, and then served on the Western Front in France, being wounded in the Battle of Pozières Heights on 29 July. He achieved promotion from corporal to sergeant and was commissioned as an officer – second lieutenant – on 7 April 1917, and to lieutenant on 28 August 1917. He attended officer training in the United Kingdom. Borella received a Military Medal for conspicuous bravery on 11 May 1917, was Mentioned in Despatches on 1 June 1917, awarded the Victoria Cross on 16 September 1918 for actions in July 1918 during a peaceful penetration operation prior to the start of the Allied Hundred Days Offensive.