Peacock enlisted in the 4th Essex Regiment (TA) on 17 March 1931 but was discharged eight months later when he joined the Royal Marines on 9 November. He served in RM detachments onboard the carrier HMS Courageous from 2 May 1933 to February 1934 and HMS Dunedin, a Danae class light cruiser from March 1935 to 2 May 1937 before being promoted to Corporal on 21 May 1937.

In 1938 he volunteered for flying training and was transferred to the RN on probation as an Acting Leading Seaman at the same time as he commencing basic flying training on No 2 Naval Pilots Course at Rochester in October 1938. On successful completion of that phase he moved to No 1 FTS, Netheravon. Other Royal Marines on his course were Lieutenants Hay and Parnell. He graduated and transferred to the Royal Navy and was promoted to Acting Petty Officer Airman on 12 August 1939.

Peacock then had a short but eventful life in the FAA. He joined 759 Squadron flying Gladiators and on the 25 November 1940 the squadron deployed to RNAS Hatston in the Orkneys and became 804 Squadron. Peacock was one of the Gladiator pilots. Shortly afterwards, he had a narrow escape on 9 January 1940 taxiing prior to take-off when his aircraft was struck by a Swordfish taking off downwind.   His Gladiator was a write-off but, fortunately, Peacock received only minor injuries. Later on 10 April 1940 Peacock was credited with a shared a victory when his flight shot down a Heinkel. He then converted to Skuas and embarked with 803 Squadron in Ark Royal initially off Norway and then in the Mediterranean.

On 6 July 1940, escorting Swordfish attacking FS Dunkerque, off Oran, he was shot down by a French fighter and ditched. Still with 803 Squadron he served briefly at RNAS Donibristle and in Formidable before being posted to Dekheila Airfield, Alexandria, in March 1941. During his tour in Egypt, it would seem Peacock became unsettled. He was no longer enjoying flying with the RN and questioned his terms of service. He requested a number of transfers that eventually led to him being granted a discharge 'Shore' on 22 August 1942.

After a brief period as an Ambulance Driver in London Peacock enlisted in the RAF on 16 March 1943 and was rapidly awarded his pilot’s badge and promoted to Sergeant the next month. He was clearly fast tracked, presumably in recognition of his previous flying experience with the FAA. He then served with various Pilot Pools, Advanced Flying Units and Operational Training Units(OTUs) as an instructor, flying Miles M.9A Masters and Hurricanes.   

He was commissioned on 25 January 1944 and posted to a Tactical Exercise Unit at Grangemouth, flying Hurricanes and Spitfires, teaching combat tactics to fighter and fighter bomber pilots. Peacock then moved to 1682 (Bomber) Defence Training Flight and, subsequently, 22 and 26 OTUs for “bomber pilot defence training” and “fighter affiliation duties”.   Peacock only flew fighters (he never trained as a bomber pilot) at all these units, whether training fighter pilots in fighter tactics, or bomber pilots and crews in defence and evasion tactics against fighters.  Peacock was released from the RAF in November 1945.