The squadron is famous for Operation Jericho on 18 February 1944, when the crews of Mosquitoes breached the walls of a Gestapo prison at Amiens, France, allowing members of the French Resistance to escape.

[2] The main role for its R.E.7s was reconnaissance,[2] while it also operated small numbers of Bristol Scout Ds[3] and a single Martinsyde G.100 as escort fighters.

[6] In February 1917, the Squadron re-equipped again, receiving the R.E.8, and gave a good account of itself in the Corps Reconnaissance role.

[2] On one day, 7 June 1917, at the beginning of the Battle of Messines, its artillery spotting was responsible for putting 72 German batteries out of action.

[10] By August 1938 it began to receive the more modern Bristol Blenheim monoplane and moved to RAF Eastchurch, also in Kent.

[14] On the outbreak of the Second World War, the squadron was converting to the Blenheim IV and so was not fully operational, mainly flying reconnaissance missions.

[2] On the same day a new 21 Squadron was formed at RAF Bodney, still with the Blenheim, but re-equipped a few months later with the Lockheed Ventura.

It flew its first operational mission using the Ventura, a low-level attack by three aircraft on railway lines near Hengelo in the Netherlands, on 3 November.

[23][24] On 6 December it took part in Operation Oyster, a large-scale attack by 2 Group on the Philips works at Eindhoven, with three of its aircraft being lost.

[27] The squadron turned to night raids on continental Europe;[28] and from the end of 1943, like the rest of 2 Group, largely concentrated on Operation Crossbow, the campaign of attacks against V-1 flying bomb sites.

[30] The squadron also carried out precision daylight raids including involvement in Operation Jericho on 18 February 1944 (where its aircraft served as a reserve force that did not need to drop their bombs),[31] a notable attack on Gestapo headquarters at Aarhus in Denmark on 31 October 1944 and in Operation Carthage, against Gestapo headquarters in Copenhagen on 21 March 1945.

The squadron, with a detachment at Laverton, Western Australia continued to support British nuclear testing.