In 1937, Gerald Slingsby Mellor was awarded a special entry engineering naval cadetship on HMS Excellent and subsequently became a midshipman. By the time the war commenced he was a Lieutenant (E). He served on HMS Tiptoe, a submarine of the third group of the "T" class, built by Vickers Armstrong, having been ordered on 22nd December 1941. She was launched on 25th February 1944 and commissioned on 12th June 1944. She was one of only 2 submarines named by Winston Churchill and has, so far, been the only ship of the Royal Navy to be so named. The Royal Navy naming committee was against the name stating that:-"it was derogatory to one of His Majesty's ships". The Prime Minister's intention was to imply that the vessel could approach the enemy silently, as if on tiptoe. Churchill had his way!

She was equipped with new radar equipment and deployed to the Far East, arriving in Trincomalee Ceylon on 1st March 1945.. Her first patrol was uneventful and centred around the west coast of Burma and the Andaman Islands and then to Fremantle Australia.Her second patrol, starting on the 6th May was to the Flores sea. On 15th May, she sank a Japanese coaster with gunfire near Dumpo Bay Sumbawa and a further coaster was sunk in the same manner on the following day in Sepeh Harbour. On 1st June, she sank a Japanese merchant cargo ship named "Tobi". The cargo ship had been about to rendezvous with an escort which launched thirteen depth charges, putting all of Tiptoes torpedos out of action and flooding her sonar system. She returned to Fremantle for repairs until 16th July when she left for her third patrol in the Sunda Strait with her sister HMS Trump.

On 31 July she attacked two small Japanese vessels but broke off when an aircraft was sighted and on 2nd August she and HMS Trump destroyed two small vessels with gunfire. They then carried out a torpedo attack against the Japanese army cargo vessel Tencho Maru and finally on 9th august they destroyed an 800 ton coastal tanker in the northern part of the Sunda Straight. HMS Tiptoe reurned to Fremantle on 21 August 1945.

The Distinguished Services Cross was awarded to Gerald Mellor for "actions, patrols, raids carried out by HM Submarines".