In 1939, Warrender joined the Royal Naval Volunteer Reserve on a whim, despite his parents' objections. After training at HMS "King Alfred", he was posted to the minelayer HMS "Manxman". He commanded a flotilla of boats in the Battle of Madagascar, and later took part in relieving the Siege of Malta. In 1943, Warrender transferred to the destroyer HMS "Savage", which formed part of an Arctic convoy for the following two years. He was "Savage"'s torpedo control officer during the Battle of the North Cape, and helped sink the German battleship "Scharnhorst", for which he was awarded the Distinguish Service Cross (DSC). In 1944, Warrender was posted to Melbourne, Australia, as a flag lieutenant to help prepare for the arrival of the British Pacific Fleet. Towards the end of the war he was stationed in Hong Kong, where he helped relieve the POW camp at Sham Shui Po and re-establish British administration.