In 1842 he served with General Pollock's army of retribution in the march upon Kabul, and shortly afterwards was appointed to a subordinate office in the military secretariat.
Owing to the absence of the head of the department on sick leave, it devolved upon Major (then Captain) Banks to make all the arrangements for the expedition which resulted in the conquest and annexation of Pegu.
Shortly after the close of the war, he accompanied Lord Dalhousie on a visit to British Burma, and subsequently became a member of the governor-general's personal staff in the capacity of military secretary.
In July 1855 he was deputed upon a confidential mission to Lucknow, to communicate to Sir James Outram, the resident, the intentions of the governor-general regarding the annexation of Oudh.
In Sir John Inglis's memorable despatch on the defence of the Lucknow residency, the death of Major Banks was noticed in the following terms:—'The garrison had scarcely recovered the shock which it had sustained in the loss of its revered and beloved general, when it had to mourn the death of that able and respected officer, Major Banks, who received a bullet through his head while examining a critical outpost on 21 July, and died without a groan.'