Northern Ireland District formed in 1922 after the partition of Ireland. There were 3 Regular Army regiments based in Ulster at the time, with District Headquarters in Belfast, Co. Antrim. The role of the district was one of internal security and defending the newly formed Northern Ireland. It came under the command of a Major General.
With the outbreak of the Second World War, the establishment of the Northern Ireland District increased. The 4 battalions based in Northern Ireland received postings overseas. Between October 1939 and April 1940, the Territorial Army 53rd (Welsh) Infantry Division arrived to replace them.
On 20th June 1940, 61st Infantry Division joined 53rd Infantry Division in Northern Ireland. The areas of Northern Ireland District underwent further changes. The District divided into Northern Area under 61st Infantry Division and Southern Area under 53rd Infantry Division. On 12th July 1940, a Lieutenant General was appointed as General Officer Commanding British Troops in Northern Ireland (BTNI). At this time, Headquarters of VI Corps set up in Belfast, Co. Antrim but was absorbed into BTNI.
On 9th May 1943, it seemed the threat of an invasion through the Republic of Ireland was no longer viable. BTNI Headquarters amalgamated with Northern Ireland District to form a combined Headquarters in Lisburn, Co. Antrim. BTNI was downgraded back to a District Establishment in 1944.