After the war Bagnold studied engineering at Gonville and Caius College, Cambridge, obtaining an MA before returning to active duty with the British Army in 1920 with the Royal Corps of Signals.

He served in Cairo and the North West Frontier, India, where he was again mentioned in dispatches.
In both of these locations, he spent much of his leave exploring the local deserts. After having read Ahmed Hassanein's "Lost Oasis" he spent one such expedition in 1929 using a Ford Model A automobile and two Ford lorries exploring the vast swathe of desert from Cairo to Ain Dalla which was an area reputed to contain the mythical city of Zerzura.

After a brief period of half-pay, he left the Army in 1935, but rejoined upon the outbreak of the Second World War

Bagnold wrote, "Never in our peacetime travels had we imagined that war could ever reach the enormous empty solitudes of the inner desert, walled off by sheer distance, lack of water, and impassable seas of sand dunes. Little did we dream that any of the special equipment and techniques we had evolved for very long-distance travel, and for navigation, would ever be put to serious use." Desert exploration Second World War On 10 June 1940 Italy declared war on the United Kingdom in alliance with Germany while Bagnold was in Cairo due to an accident involving a troopship collision that he was on interrupting his journey elsewhere. Upon hearing the news and realizing that North Africa was about to become a theatre of war, he requested an interview with General Archibald Wavell, Commander-in-Chief Middle East. Having secured it, Bagnold suggested that Wavell use his knowledge of the terrain in North Africa to establish a mobile scouting force for desert operations against the Italian Armed Forces in Libya, which Wavell was charged with defeating in the field. During the conversation Wavell asked Bagnold what he would do if he found that the Italians were not doing anything beyond the Libyan coast in the desert interior. Bagnold replied that the new unit that he had in mind might be able to commit "acts of piracy on the high desert". Wavell granted Bagnold authority to form a unit along these lines, with it being constituted in July 1940 with the name Long Range Desert Group (L.R.D.G.). After assembling its first formation, Bagnold was the L.R.D.G.'s commanding officer until August 1941, when he handed over command to Guy Prendergast on being promoted to the post of Inspector of Desert Troops. In October 1941 he was promoted to the post of Deputy Signal Officer-in-Chief Middle East, with the temporary rank of Brigadier and worked on camouflage and deception operations.

On 7 June 1944 Bagnold retired from the British Army with the end of military operations in North
Africa after the Axis powers' defeat in that theatre. and returned to his scientific interests, being
elected to a Fellowship of the Royal Society in the same year.
After the war Bagnold continued to work in the field of the geological science, and he published
academic papers into his nineties. He made significant contributions to the understanding of
desert terrain such as sand dunes, ripples and sheets. He developed the dimensionless "Bagnold
number" and "Bagnold formula" for characterising sand flow. He gave a constitutive relation for a
suspension of neutrally buoyant particles in a Newtonian fluid.