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In 1799, Henry Darlot, a clerk in the Foreign Section, was chosen for the position of Army Postmaster. He was the first to accompany the Army overseas when he joined them in Holland to facilitate delivery, collect letters, and protect the revenue. In 1854, Edward Smith of the Inland Letter Office was sent to Constantinople as postmaster to H M Forces. He, along with three assistants and seven sorters, handled 450,000 letters a month to and from troops between Britain and the Crimea (via France).

The origins of the Post Office Rifles Association stem from the Fenian troubles in 1867. Sixteen hundred Post Office staff were sworn in as special constables to protect key installations, including post offices. With the passing of the danger the following year, a number of the special constables asked if a Post Office Volunteer Corps could be formed. The War Office officially sanctioned the proposal, and the 49th Middlesex Rifle Volunteers were formed in 1868. The title of the regiment was changed to the 24th Middlesex (Post Office) Rifle Volunteers in 1880. In 1882, the Army Postal Corps was formed from the members of the Middlesex Volunteers to run the Army's postal service. A sister unit, the Field Telegraph Corps, was formed from postal workers in 1883 becoming 'I' Company of the 24th Middlesex. Both units were embodied in the reserve of the Royal Engineers in 1884, although still attached to the 24th Middlesex for drill and discipline.

The Boer War saw eleven officers and 624 other ranks serve with the Army Postal Corps between 1899 and 1902. Their base in Capetown connected with many temporary Field Post Offices and five travelling post offices. 68.9 million letters and newspapers and 1.4 million parcels were delivered to the troops. In an average December week, 789 bags of mail were received in Capetown, and in the busiest week of the war 643,000 letters and newspapers and 33,967 parcels were delivered.

In 1908 the 24th Middlesex Volunteers became the 8th Battalion of the City of London Regiment (Post Office Rifles) on formation of the Territorial Force. This battalion and its two sister battalions (2/8th and 3/8th raised during the First World War) were largely made up of Post Office personnel but no longer had any direct links with the Army Post Office Corps. In 1913 the Army Post Office Corps became the Royal Engineers Postal Section and the Telegraph Reserve became the Royal Engineers Signal Section. At the outbreak of the First World War a Base Post Office was set up at Le Harve, whence army mail went to an Advanced Base Post Office and then to Field Offices and Branches. Primary sorting in the UK was done from the Home Depot. All mail below 4 oz. from servicemen was carried free, with letters to them addressed "c/o GPO, London" then sorted by code to preserve secrets of military layout. Censorship was operated by military authorities. The shortage of men led to a reduction in the number of deliveries and 35,000 women were temporarily employed. Subdivision of the London Postal Districts was introduced in 1917 to aid the women sorters.

The Second World War presented an even greater challenge, with mobile fronts all over the world and enemy air attacks at home. At the outbreak of the war the Post Office was the largest employer in the country, and by mid-war nearly a third had volunteered for active service. Fifty thousand staff were members of the Post Office Home Guard, who were detailed to defend telephone and telegraph facilities in case of invasion. Postwomen were again taken on to fill the gaps, often working long shifts. Mobilisation then evacuation caused millions of people to change address, greatly increasing the volume of mail and the percentage of insufficiently addressed letters. Payment of pensions and allowances greatly increased, as well as new tasks like the distribution of millions of ration books, public information leaflets and permits. The blackout made sorting, delivery and station work very difficult, and the need to blacken the glass roofs of sorting offices etc. meant that many sorters and telephonists worked all the time by artificial light.

Air raids brought large scale destruction of Post Office buildings, mostly in major urban centres, telephone cables had to be repaired or re-routed as quickly as possible, destruction at major railway termini often meant improvised re-routing of mails, and bomb damage sometimes made letter delivery hazardous and difficult, when the numbers on houses, or the occupants, or even the houses themselves might disappear overnight. In September 1940 23 post offices were destroyed in one night. During the Blitz post office buildings had their own fire-fighting teams composed of staff, often very efficient in preserving its buildings.

The Army Post Office was based at Nottingham in a former textile factory. Army and RAF mail was handled there by hundreds of WRAC women, plus men unfit for active service and GPO officials who had volunteered. Insufficiently addressed letters were also handled at Nottingham by the Post Office. Naval mail for ships in foreign waters was handled by Wrens at King Edward Building, London, then by the Admiralty at Reading. Hostilities in the Mediterranean posed particular difficulties for getting mail to British forces in the Middle and Far East. The sea route around the Cape added 12,000 miles to the journey, a 3 month delay, and aircraft space was at a premium. Microphotography offered a solution and the airgraph service was introduced in 1941. Some 330 million airgraphs were sent until the service ended in July 1945. At first airgraphs and air letters were for military use only, but were then made available to civilians. By 1945 600,000 civilian air letters per week were being despatched to 33 different countries.

Prisoner of war mail was despatched abroad by the Post Office. Between 1941 and 1945, 26,250,000 parcels (both Red Cross parcels and parcels from next-of-kin) were sent from Mount Pleasant via Portugal to Geneva (despite difficulties in getting around or across enemy territory) where they were transmitted forward by the International Red Cross. About 200,000 letters per week were sent to POWs from Britain by air to Lisbon, where an exchange system operated with mail from Germany for German prisoners in Britain and Canada. The Post Office was in effect a fourth service, vital to the survival of the state and performed its duty well despite labour shortage, the need to recruit and train inexperienced staff, and enemy attack.