Ralph Alger Bagnold was born in Devonport in 1896, the son of Colonel A.H. Bagnold, Royal Engineers and grandson of Major-General M. E. Bagnold of the East India Company. He was educated at Malvern College and the Royal Military Academy, Woolwich, before his commission in the Royal Engineers with the rank of 2nd Lieutenant in 1915. During the First World War he served at Ypres, the Somme and Passchendaele. He rose to the rank of Captain.

In 1919 Bagnold returned to England and entered Gonville and Caius College Cambridge to study for the Engineering Tripos. He completed his degree in two years and returned to the army in 1921. He was posted to the 5th Division Signal Company, then serving in Ireland. On the transfer of the signal units from the Royal Engineers to a new Royal Corps of Signals, Bagnold was sent to the Signal Training Centre at Maresfield Park, Sussex, first as instructor in electricity, then as Chief Instructor.

In 1926 Bagnold was posted to Egypt. He served for two years before promotion to the rank of Major took him to the North-West Frontier of India 1928-31 where he commanded the Waziristan Signals. After a period in England as Chief Instructor at the School of Signals at Catterick, Bagnold was posted to the Far East as Officer Commanding Signals, China Command in 1933. He retired from the army in 1935 on health grounds.

Bagnold's time in Egypt instilled in him a fascination with desert exploration. He went on expeditions with fellow officer into Sinai [Egypt], Transjordan [later Jordan] and the Libyan Desert and, even after his posting to India, returned to North Africa to lead expeditions in 1929, 1930 and 1932. The 1932 expedition was the most adventurous, covering the unexplored frontier between Chad and northern Sudan. This and the earlier explorations are recounted by Bagnold in Libyan Sands. Travel in a dead world, first published by Hodder & Stoughton in 1935. In recognition of his explorations in North East Africa Bagnold received the Gold Medal of the Royal Geographical Society in 1934.

During his desert expeditions Bagnold had become interested in the physics behind the creation and movement of sand dunes. On retirement from the army he began scientific research in C. M. White's laboratory at Imperial College London using a home-made wind tunnel. This work culminated in 'The physics of blown sand and desert dunes', first published by Methuen in 1941. This became the standard text-book on the subject and was still in print when Bagnold died in 1990.

On the outbreak of the Second World War Bagnold was recalled to the army. He was originally posted to Kenya but his troopship collided in the Mediterranean en route and put in for repair in Egypt. General Wavell requested that Bagnold remain in Egypt. In 1940, with General Wavell's support, Bagnold founded the Long Range Desert Group (LRDG), a small motorised force which undertook reconnaissance and raids deep into enemy held territory. The LRDG met with considerable success and made a valuable contribution to the Allied victory in North Africa and Bagnold received the OBE (Military) for the part he played in establishing the Group. He handed over command in July 1941 and joined the General Staff in Cairo with the rank of Colonel. Bagnold was later promoted to Brigadier, serving as Deputy Officer-in-Chief of Signals in the Middle East.

Bagnold returned to England in 1944. He was married in 1946 to Dorothy Plank. In 1947 Bagnold was offered and accepted the post of Director of Research at Shell Refining and Marketing Company, based at the new Thornton Research Centre, near Chester. He resigned in 1949 in order to be able to concentrate on research into the transport of solids by a stream of water, at Imperial College London.

He served as consultant on several projects, most notably for the US government from 1958. This arose from an approach in 1956 by L. B. Leopold, Head of the Water Resources Division of the US Geological Survey, to undertake collaborative work on the annual rate at which rivers transport solids. Bagnold's experiments demonstrated that transport rate is an inherent function of flow-depth, a fact previously unknown. He remained an authority on the transport of blown sand and in 1977 was invited by NASA to be key-note speaker at a meeting on the desert landscapes of Earth and Mars.

In later years Bagnold also began to study patterns of random distributions, work which had its origins in observations he had made in 1927. The paper Bagnold published on this in the Proceedings of the Royal Society in 1983 prompted a friendly letter from B. B. Mandelbrot, a pioneer of chaos theory.

Bagnold received many honours and awards for his scientific work. In 1944 he was elected to the Fellowship of the Royal Society. In 1969 he received the G. K. Warren Prize of the US National Academy of Sciences, and in 1970 the Penrose Gold Medal of the Geological Society of America and the Wollaston Medal of the Geological Society of London. Bagnold was elected a Fellow of Imperial College in 1971. He received an Honorary D.Sc. From the University of East Anglia the following year, was elected a Foreign Honorary Member of the American Academy of Arts and Sciences in 1974, and in 1978 was awarded the Sorby Medal of the International Society of Sedimentologists and an Honorary D.Sc. From the University of Aarhus in Denmark. Shortly before his death in 1990 Bagnold was awarded the James Alfred Ewing Medal of the Institution of Civil Engineers and Royal Society.