Lyall joined the Royal Marines on 28 January 1935 at the Depot RM, Deal. His Regimental Number was: Chatham/X1394. After recruit training he attended gunnery courses before being drafted to HMS Ramillies, a Revenge class battleship. After further upgrading gunnery courses he was promoted to Corporal on 3 December 1937 and posted briefly to HMS Dragon, a Danae class cruiser and then HMS Resolution, another Revenge class battleship and volunteered for flying training. On 29 April 1939 he was transferred to the Royal Navy on probation as an Acting Leading Seaman to commence pilot training on No 4 Naval Pilots Course. Other Royal Marines on his course were; Lieutenants Cheesman, Jolliffe, Marryat and Wright, Corporals Chapman, Jaggard and Yarde; the latter did not qualify. His Elementary Flying Training took place at Gravesend and Rochester before graduating to No 7 FTS at Peterborough. He completed flying training on 3 December1939 and was promoted to Acting Petty Officer Airman the next day; his FAA number was FX 97088. Immediately he commenced seaplane and operational training at RNASs Lee-on-Solent, Donibristle and Ford.

After his seaplane courses Lyall's service record includes two unusual postings: HMS Daedalusfor 'Bishop force' 20 June to 1 July 1940 and 'Profit Force' 1 July to 11 November 1940. For the last ten days of June 1940, when 'Bishop Force' appears in Lyall's record, 701 Squadron was either at RNAS Donibristle or embarked in Argus.  Given the juxtaposition of dates, it is possible that, for Lyall, 'Bishop Force' morphed into 'Profit Force'. 701 Squadron actually embarked in Argus from Donibristle on 23 June and disembarked at Reykjavik on 1 July 1940. Its role together with other forces was to forestall a German occupation of Iceland. The squadron returned to RNAS Donibristle on 26 October 1940. HMS Force Profit was the most unusual 'commissioned ship' name given to the RNAS aircraft based in Iceland, the men being borne for pay on the books of RNAS Lee-on-Solent.

After 'Force Profit' Lyall had yet another unusual draft when he was posted to HMS Mentor and also confirmed in the rank of PO Airman on 13 February 1941. During the Second World War, the Royal Navy took over Lews Castle, near Stornaway, as accommodation for air and ground crews of 700 Naval Air Squadron. The squadron operated a detachment of six Walrus aircraft from a slipway at Cuddy Point in the Castle's grounds and the base was designated HMS Mentor.

Lyall joined HMS Edinburgh,from 1 June 1941 to 13 January 1942. Edinburgh was a Town class light cruiser which carried two Walrus aircraft and during the above period was engaged in operations in the Denmark Strait and escorting convoys to Gibraltar and Malta. He was then drafted to 749 Squadron, HMS Goshawk, RNAS Piarco, Trinidad, on 24 April 1942 but was tragically killed on 12 August 1942 when at night his Walrus aircraft crashed into the sea in the Gulf of Paria.

PO Airman David Lyall and his crew, Acting Leading Airmen Patrick Fagan and John Kirton, are commemorated on the Lee-on-Solent War Memorial. As the principal base of the FAA during the Second World War, Lee-on-Solent was chosen as the site for the memorial to almost 2000 men who died during the war and have no known grave.