After completing trials under the command of Lieutenant Commander E.D. Norman, Graph was placed under the command of Lieutenant Peter Barnsley Marriott, who had assisted in the trials.
She departed from Holy Loch for her first Royal Navy war patrol on 8 October 1942, with the intention of patrolling the Bay of Biscay.

On the afternoon of 21 October 1942, about 50 nautical miles (90 km; 60 mi) north-north-east of Cape Ortegal (44°31′N 7°25′W), Graph dived to evade a German Focke-Wulf Fw 200 long-range patrol aircraft.
A loud hydrophone contact made Marriott believe a nearby submarine had likewise dived and, 12 minutes later, he observed its conning-tower against the setting sun.
After pursuing the German boat, Graph fired four torpedoes.
Explosions were heard, and also banging noises, leading the British to believe they had hit the other submarine and the banging noises were caused by her breaking up as she sank.
In early 1943, Marriott was awarded the Distinguished Service Order for "great courage, skill and determination in a successful submarine patrol".

After the war, examination of German records showed the submarine attacked was the U-333, badly damaged after being rammed by the Flower-class corvette HMS Crocus off the coast of West Africa.
German lookouts had seen the torpedoes' tracks, enabling U-333 to evade the torpedoes; the torpedoes then self-detonated for unknown reasons.
The commander of U-333 was Peter-Erich Cremer.
In his post-war account of the attack, he suggested the rattling and banging noises Graph's crew had heard were due to the severe damage previously inflicted on U-333.
His route back to France closely hugged the Spanish coastline, a pattern followed by other U-boats, and he had also believed that Marriott was aware of this and had been lying in wait.

Graph's second war patrol was from 19 November 1942 to 8 December 1942, also in the Bay of Biscay.
During the patrol, she was ordered to intercept the Italian cargo ship Cortellazzo, which was passing through the area en route from Bordeaux to Japan with a cargo of 2000 tons of machinery.
However, Graph failed to find the ship and the patrol was without incident.
Cortellazzo was sunk within days by HMS Redoubt, one of the destroyer escorts of an Allied troop convoy, after Redoubt had ordered Cortellazzo' crew to abandon ship and had picked them up.

Graph departed from Lerwick on her third war patrol on 24 December 1942 with three other Royal Navy submarines; their task was to patrol the waters off the coast of Norway in the area of Altafjord.
At 1 am on 1 January 1943, at the position 70°51′N 21°56′E, she sighted the German heavy cruiser Admiral Hipper, returning from the unsuccessful attack on convoy JW 51B, better known as the Battle of the Barents Sea.
Hipper was too far off and travelling too fast to be attacked.
At 4:23 am, Graph sighted two German destroyers moving erratically and at low speed.
Graph closed to a range of 7,000 yards (6,400 m) and fired four torpedoes.
Again, explosions convinced the British that hits had been scored and a destroyer probably sunk, but again all the shots had missed.
Graph returned to Lerwick on 13 January 1943.

Graph undertook no further war patrols.
In 1943, she was assigned for training duties and command of her passed to Lt.
D. Swanston.
Peter Marriott went on to command HMS Stoic, where he was awarded a DSC.
After completing trials under the command of Lieutenant Commander E.D. Norman, Graph was placed under the command of Lieutenant Peter Barnsley Marriott, who had assisted in the trials. She departed from Holy Loch for her first Royal Navy war patrol on 8 October 1942, with the intention of patrolling the Bay of Biscay.
On the afternoon of 21 October 1942, about 50 nautical miles (90 km; 60 mi) north-north-east of Cape Ortegal (44°31′N 7°25′W), Graph dived to evade a German Focke-Wulf Fw 200 long-range patrol aircraft. A loud hydrophone contact made Marriott believe a nearby submarine had likewise dived and, 12 minutes later, he observed its conning-tower against the setting sun. After pursuing the German boat, Graph fired four torpedoes. Explosions were heard, and also banging noises, leading the British to believe they had hit the other submarine and the banging noises were caused by her breaking up as she sank. In early 1943, Marriott was awarded the Distinguished Service Order for "great courage, skill and determination in a successful submarine patrol".
After the war, examination of German records showed the submarine attacked was the U-333, badly damaged after being rammed by the Flower-class corvette HMS Crocus off the coast of West Africa. German lookouts had seen the torpedoes' tracks, enabling U-333 to evade the torpedoes; the torpedoes then self-detonated for unknown reasons. The commander of U-333 was Peter-Erich Cremer. In his post-war account of the attack, he suggested the rattling and banging noises Graph's crew had heard were due to the severe damage previously inflicted on U-333. His route back to France closely hugged the Spanish coastline, a pattern followed by other U-boats, and he had also believed that Marriott was aware of this and had been lying in wait.
Graph's second war patrol was from 19 November 1942 to 8 December 1942, also in the Bay of Biscay. During the patrol, she was ordered to intercept the Italian cargo ship Cortellazzo, which was passing through the area en route from Bordeaux to Japan with a cargo of 2000 tons of machinery. However, Graph failed to find the ship and the patrol was without incident. Cortellazzo was sunk within days by HMS Redoubt, one of the destroyer escorts of an Allied troop convoy, after Redoubt had ordered Cortellazzo' crew to abandon ship and had picked them up.
Graph departed from Lerwick on her third war patrol on 24 December 1942 with three other Royal Navy submarines; their task was to patrol the waters off the coast of Norway in the area of Altafjord. At 1 am on 1 January 1943, at the position 70°51′N 21°56′E, she sighted the German heavy cruiser Admiral Hipper, returning from the unsuccessful attack on convoy JW 51B, better known as the Battle of the Barents Sea. Hipper was too far off and travelling too fast to be attacked. At 4:23 am, Graph sighted two German destroyers moving erratically and at low speed. Graph closed to a range of 7,000 yards (6,400 m) and fired four torpedoes. Again, explosions convinced the British that hits had been scored and a destroyer probably sunk, but again all the shots had missed. Graph returned to Lerwick on 13 January 1943.
Graph undertook no further war patrols. In 1943, she was assigned for training duties and command of her passed to Lt. D. Swanston. Peter Marriott went on to command HMS Stoic, where he was awarded a DSC.
HMS Graph