After his initial period of general service as a Lieut. in HMS Resolution mostly off the coast of Spain during that Civil War, he took up flying in January 1938. At the outbreak of war he was already serving in a first line fighter squadron (803) in the Ark Royal.
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On 14 September 1939 an S.O.S. was received from a merchant ship, SS Fanad Head some three hundred miles off the coast of Ireland. The ship was being attacked by a German U-boat (U 30). Ark Royal was about two hundred miles to the north east but hurried to her assistance and flew off 3 Skuas of 803 Sqn. (Lt Cdr. Campbell, Lt. Griffiths and Lt. Thurstan). At about this time another U-boat, U39 sighted the Ark Royal and fired a salvo of three torpedoes at her but fortunately these were sighted and the Ark took avoiding action just in time although one exploded in her wake. This alerted the destroyer escorts and the U-boat was blown to the surface and her crew captured, the U-boat sinking a few minutes later (the first one of the war).
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Meanwhile the three Skuas had found the Fanad Head about a hundred miles away to the south west with her passengers and crew in lifeboats with the U-boat on the surface trying to sink her with gunfire. On arrival of the Skuas she crash-dived leaving two of her crew to swim. Thurstan and Griffiths dived into the attack but in pressing home their attacks they were both hit by the blast from their own bombs. This damaged their tailplanes and both aircraft crashed into the sea. The airgunners were both lost but with great good luck the two pilots survived and swam to the abandoned Fanad Head. Shortly afterwards the U-boat surfaced again and they were taken prisoner by the crew who came on board to search the ships' papers. As they were being taken on board the U-boat they were unsuccessfully attacked by Swordfish from the Ark and Griffiths and Thurstan both became the first P.O.W.'s.
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The bombs used in this attack were 112 lb. AS. but were apparently wrongly fused and it was a long time before this was discovered although Griffiths tried to get a message across in a letter he wrote on 2 November 1939 from his P.O.W. camp OFLAG IX A ( Spangenberg Castle). Guy Griffiths was in the same P.O.W. camp as Wings Day until the latter was moved to Stalag Luft 1. He was moved from Spangenberg Castle to DULAG LUFT in 1940 and took part with 17 others, including Wings Day and Major "Birdie" Partridge, in the first escape by tunnel. All were recaptured.
On return to duty after being liberated at the end of hostilities he returned to the Fleet Air Arm and was posted to 703 Sqdn. at Thorney testing new weapons and equipment. Here he flew one of the first Sikorski Hoverfly Helicopters in the UK and was undoubtedly the first RM pilot to fly a helicopter regardless of the claims of others. After qualifying in Instrument Flying he also passed the Fighter Leaders course and then returned to 703 Sqn as a trials pilot with the joint RN and RAF Air Sea Warfare Development Unit.
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In May 1948 he was recalled to Corps Duty and posted as OCRM HMS Howe until 1950 when he qualified as A.L.O. at the School of Land Air Warfare at Old Sarum. He then served on board HMS Glory in Korean waters with the 14 C.A.G. with No 67 CBAL Section. Whilst there he was instrumental in discovering a Mig 15 lying in shallow water behind the enemy lines so that the US Navy were able to recover it for examination. In July 1950 he was posted to Eastney where he became Editor of the Globe and Laurel and also Civil Defence Staff Officer.

He retired in May 1958 and later joined the Hospital Administration Service