III Corps was formed on 25 October 1939. It opened in France in April 1940 taking command of the 5 Infantry Division on 8 April 1940, the division transferring from II Corps. The 42 (East Lancashire) Division arrived in France on 12 April 1940 to join III Corps on 29 April.

The 44 (Home Counties) Division arrived in France on 1 April 1940 and joined III Corps on the same day. On 9 May 1940, when the German invasion of Western Europe commenced, the corps had under command the 42 (East Lancashire) Infantry Division and 44 (Home Counties) Infantry Division, as the 5 Infantry Division had passed to the command of G.H.Q. B.E.F. on 1 May 1940.

In June 1940, following evacuation from Dunkirk, the corps moved to the North West of England. It took under command the 38 (Welsh) Infantry Division on 15 July 1940, but this formation left on 29 October 1940 to transfer to G.H.Q. Home Forces. On the same date, the 5 Infantry Division transferred to join the corps from Scottish Command. The 5 Infantry Division remained under command until 31 March 1941 when it moved to Northern Ireland.

In late April 1941, the corps headquarters moved to Northern Ireland, where it took command of the 5 Infantry Division and 61 Infantry Division on 1 May 1941. The 5 Infantry Division left the corps on 16 January 1942 to move to India. In the meantime, the 59 (Staffordshire) Infantry Division had moved to Northern Ireland to join III Corps on 10 November 1941. The headquarters appear to have left Northern Ireland on 31 May 1942 to move to Iraq. It later moved to Syria, and in 1944, Greece.