139th (Mixed) Heavy Anti-Aircraft Regiment was an air defence unit of Britain's Royal Artillery formed during World War II.

The ATS was by law a non-combatant service, but it was decided that Defence Regulations permitted the employment of women in anti-aircraft (AA) roles other than actually firing the guns.

It was then joined on 15 December by the following batteries:[2][3][5] In January 1942 the new regiment was assigned to 62 AA Brigade, responsible for the defence of Leeds and Sheffield in West Yorkshire under 10th Anti-Aircraft Division.

In February, the regiment was joined by two additional batteries:[5][6] In the spring of 1942, a new phase in the air campaign began with the so-called Baedeker Blitz mainly directed against undefended British cities.

In 10th AA Division's area, York was accurately hit on 28 April, Hull on 19 May and 31 July, and Grimsby on 29 May, but the strongly defended towns of West Yorkshire were not attacked.

[5] As new units joined AA Command, more experienced ones were being posted away to train for service overseas, particularly for the planned invasion of North Africa (Operation Torch).

[7][11] In the summer of 1943, the Luftwaffe made a few raids against East Coast towns including Hull and Grimsby, and 5 AA Group, which was now responsible for the region, shuffled some of its units.

A second campaign of air-launched missiles coming in from the North Sea led to a second redeployment by AA Command to East Anglia, but again 139th (M) HAA Rgt was unaffected by the moves.

AA Command's experience had shown that the power-operated, remotely controlled static Mk IIC 3.7-inch HAA gun, which had power traverse and automatic fuze-setting, accompanied by the most sophisticated Radar No 3 Mark V (the SCR-584 radar set) and No 10 Predictor (the all-electric Bell Labs AAA Computer), were required to deal effectively with V-1s, but 21st Army Group's mobile HAA units did not have experience with this equipment.

The regiment's advance parties arrived in Antwerp on 18 November and spent an uncomfortable week in temporary accommodation under V-1 attack before moving on to Brussels.

Unlike the anti-Diver guns firing over the English Channel or North Sea, VT Proximity fuzes could not be employed by the HAA batteries at Brussels because of the risk of casualties to troops and civilians under the missiles' flightpath.