The Devonshire Regiment raised the 7th, 8th and 9th Territorial Army battalions, in addition to the 4th, 5th and 6th, all of which (except the 5th, which was converted pre-war into 86th Anti-Tank Regiment, Royal Artillery) were serving in the 45th (Wessex) Infantry Division on the outbreak of war. However, none of these units, save the 4th Battalion, saw active service outside of the United Kingdom and were used mainly for home defence, training or supplying the other battalions of the regiment with infantry replacements and served with many different brigades and divisions such as the 80th Infantry (Reserve) Division. The 4th Battalion was sent, in May 1940, to Gibraltar to join the 2nd Gibraltar Brigade[54] and returned to the United Kingdom on 28 December 1943 and eventually joined the 164th Brigade, 55th (West Lancashire) Infantry Division before finally ending the war in the 183rd Infantry Brigade, 61st Infantry Division. The 6th Battalion was transferred to the 141st Brigade, 47th Division.

The 5th and 7th Battalions were converted to anti-tank units, becoming 86th, and 87th Anti-Tank Regiments, Royal Artillery respectively. The 86th Anti-Tank Regiment was a corps-level Anti-tank unit with XII Corps in the North West Europe Campaign, while the 87th Anti-Tank Regiment was active in North Africa with the British 1st Army before being disbanded in 1944.

The 50th (Holding) Battalion was raised in 1940 and renumbered the 12th Battalion the same year and spent most of its time on home defence anticipating a German invasion. In June 1943, due to the huge expansion of the British Army's airborne forces, the battalion was transferred to the 6th Airlanding Brigade, part of the 6th Airborne Division, and were converted into glider infantry, trained to enter battle by glider.

The battalion landed in Normandy in the late afternoon of 6 June 1944 in Operation Mallard. The battalion also fought in the Battle of Breville, and served throughout the Battle of Normandy but as normal infantrymen. The battalion remained in Normandy until August 1944 where it participated in the breakout from the beachhead. The battalion, along the rest of 6th Airborne, was withdrawn to England in early September where they received new replacements, equipment and continued training. In December 1944 they fought briefly in the Battle of the Bulge but the outcome was already decided before the division arrived.

The battalion crossed the River Rhine in Operation Varsity in March 1945 alongside the U.S. 17th Airborne Division. The battalion ended the war by the River Elbe. Throughout its time in 6th Airlanding Brigade, the battalion was allegedly nicknamed the Swedebashers by the men in the other battalions (1st RUR and 2nd OBLI), due to the battalion being commanded by a regular army officer but nearly all the officers and men of the 12th Devons had enlisted for hostilities-only.