The intended purpose of the army was to carry out the breakthrough phase of the Somme offensive once General Sir Henry Rawlinson's Fourth Army had captured the German front-line trenches. For this role Gough was provided with the three British cavalry divisions and in June he was allocated an infantry corps of three divisions to support the advance. The army was nicknamed "Gough's Mobile Army", a name which had first belonged to the 7th Division when Gough had been its commander. However on 21 June this infantry corps was withdrawn to GHQ reserve and the following day Gough's three cavalry divisions were placed under Fourth Army control, making the Reserve Army little more than a glorified corps.
In the evening of the first day on the Somme, 1 July 1916, the British Commander-in-Chief General Sir Douglas Haig relieved Rawlinson's Fourth Army of responsibility for the northern sector, placing the VIII and X Corps under Gough's command. Control of the cavalry divisions was retained by the Fourth Army. Gough would not officially assume his new command until 7am on 2 July but he immediately cancelled the orders for VIII Corps to resume the failed attack on Beaumont Hamel, thereby no doubt saving many lives.
The Reserve Army's first major battle was the Battle of Thiepval Ridge which began on 26 September. For much of October the Reserve Army carried out a series of attacks known as the Battle of the Ancre Heights and on 30 October the Reserve Army was renamed the Fifth Army.