Geoffrey was born in Chertsey, Surrey, the only son of Ernest Frederick Castellain and his wife Annie H James. He had a younger sister. The Castellains were cotton brokers in Liverpool for several generations.

He was educated at Westminster where he was a Scholar and matriculated in 1938. He was a Scholar of Christ Church from 1938 until 1940 when he graduated with a 2nd in Classical Mods.

He was a cadet at Sandhurst and was gazetted 2nd Lieutenant in the 16th/5th Lancers on 8 March 1941.

He was a Lieutenant with ‘A’ Squadron 2nd Special Air Service in 1944 when he was part of Operation Pistol.

Operation Pistol was an SAS mission with the objective to parachute four teams from ‘A’ Squadron 2nd Special Air Service behind the German lines around the Alsace-Lorraine region of France. Once on the ground, they would attempt to disrupt the road and rail networks between Metz and Nancy and the approach to the Rhine plain. Because of poor weather conditions, one team returned to England, another team landed in the middle of a German unit and the other two teams achieved some success before returning to Allied lines in early October.

The 51 men left RAF Keevil by Shorts Stirling aircraft on 15 September. They were divided into four groups, to be parachuted into four different drop zones. Once on the ground, the plan was for each of the groups to be divided into two smaller groups to cover a larger area. The 'B1' group would be west of Ingwiller and the 'B2' group in the Saargemund area.

The 'B' group landed 7 miles from their drop zone.  The 'B1' group commander, Lieutenant Birnie, was captured on 17 September and died in a prisoner of war camp after an allied air raid. Also part of the 'B1' group were Corporal Gilbert Voisin who was captured on 1 October and Private Gerhard Wertheim, who was captured in September. He was executed by the Germans

Geoffrey was in command of the ‘B2’ Group the other members of which were Cpl. J Laybourne. Pte. H Arnold. Pte. J Stainton., Pte. C Ashe and  Pte. S Wrobel. Private Ashe was captured on 23 September and executed at Gaggenau in Germany.

Geoffrey died of wounds on 12 October.

He is buried with nine other British soldiers in the churchyard at Moussey, Vosges. His grave is number 7.

He is commemorated on the Special Air Service S.O.E. GHQ Liaison Regiment War and on the Memorial at the National Memorial Arboretum at Alrewas, Staffordshire.