The regiment formed part of 26th (London) Anti-Aircraft Brigade defending the London Inner Artillery Zone. Anti-Aircraft Command mobilised on 24 August 1939, and so 86th (HAC) HAA Rgt was already manning static gunsites at places like Primrose Hill and Finsbury Park when war was declared on 3 September. The regiment served in the defence of the capital throughout The Blitz. It became a mobile unit in 1942 and was one of the first units to land on D-Day, with Regimental Headquarters commanding a composite AA Assault Group on Juno Beach. During the Normandy Campaign and subsequent advance into Belgium the regiment's 3.7-inch HAA guns were sometimes used to engage ground targets. During the winter of 1944–45 its guns and radar defended Brussels and Antwerp against V-1 flying bombs (known as 'Divers').