Geoffrey Milner was born in Dewsbury, Yorkshire on 26 September 1918, the son of Geoffrey Milner (an RAF Officer) and Kathleen Isabel Milner (née Nicholson). He joined the Royal Navy as a Cadet, was promoted to Midshipman on 1 September 1936 and appointed to the battleship HMS RODNEY on the same date. He was promoted to Acting Sub Lieutenant on 1 September 1938 and was confirmed in the Rank of Sub Lieutenant on 1 January 1939.

He joined HMS DOLPHIN for the Submarine Course on 11 September 1939 and, on completion of training on 28 October 1939, he was appointed to the Submarine Depot Ship HMS MEDWAY and then, on 15 May 1940, to HMS PARTHIAN as Third Hand. He then joined HMS L26 as First Lieutenant on 11 October 1941, followed by HMS TRIBUNE as First Lieutenant on 11 January 1942, and HMS P614 as First Lieutenant on 18 February 1942.

Geoffrey Milner completed his Commanding Officers Qualifying Course on 3 May 1942, and he was then appointed to HMS L27 in command on 1 July 1942. He was then appointed to HMS P223 in command on 7 September 1942. P223 conducted one war patrol as HMS P223, before being renamed HMS SEA NYMPH. SEA NYMPH then conducted two further war patrols before Geoffrey Milner was appointed to HMS SIMOOM in command to date 15 April 1943.

SIMOOM was sent to Gibraltar conducting a war patrol in the Bay of Biscay on the way. Moving on to Algiers further patrols were conducted to the West, East and North of Corsica and Sardinia, and SIMOOM also functioned as a beacon for the allied landings at Salerno. On 2 November 1943, Geoffrey Milner was ordered to conduct a patrol in Greek waters. On 5 November he was diverted to a patrol area near the entrance to the Dardanelles. SIMOOM failed to return to harbour and was presumed to have been lost with all hands on or about 19 November 1943