Ernst Reuning studied geology at the Technical University, Darmstadt, and the University of Giessen, where he was an assistant to the professor of geology, F.W. Erich Kaiser*, and was awarded the degree Doctor of Philosophy (Dr Phil). An early publication by him dealt with "Diabaasgesteine an der Westerwaldbahn Herborn-Driedorf..." (Neues Jahrbuch für Mineralogie, 1907, 70 pp). After working for some time as a surveyor in India he was employed in 1909 as a geologist by the Deutsche Diamanten Gesellschaft in German South West Africa (now Namibia), under Dr Heinrich Lotz*. His brief was to prospect for diamonds along the coast between Lüderitz and the Orange River, following the discovery of diamonds near Lüderitz in 1908. Believing that the diamonds had been carried to the coast by rivers, he and the prospector Georg Klinghardt dug prospecting trenches on the northern bank of the Orange River near its mouth in 1912, but stopped when he was recalled to Lüderitz. Sixteen years later one of the world's richest diamond deposits was discovered in the area. Reuning next led an exploratory camel ride from Lüderitz northwards along the coast to Walfish Bay in 1912, through a vast and at the time unknown dune-field. The journey was described, with a map on a scale of 1:400 000, in "Eine Reise längs der Küste Lüderitzbucht-Swakopmund im Februar-März 1912" in the Mitteilungen aus den deutschen Schutzgebieten (1913). He was one of the very first geologists to investigate the Namib desert and his work in this extremely inhospitable environment required great courage and endurance.
Shortly before the outbreak of World War I (1914-1918) Reuning became a director of the Deutsche Kolonial Gesellschaft für Südwestafrika, with its local headquarters in Swakopmund. During the early nineteen-twenties he undertook an investigation of the relationship of the numerous granite intrusions and mineralised pegmatites in central Namibia, which led to papers in Geologische Rundschau and other German journals during 1922-1925. He gave particular attention to the tungsten, molybdenum, copper and gold-bearing ores of the Natas Mine (some 100 km west of Rehoboth). In another paper in the Centralblatt für Mineralogie (1925) he discussed occurrences of elemental sulpher along the Swakop River near Richthofen and their possible relation to fish mortality at Walfish Bay. Meanwhile he had published the first geological reconnaissance map of central Namibia, Geologische Uebersichtkarte des mittleren Teils von Südwestafrika (scale 1: 1 000 000) in 1923.
In collaboration with the palaeontologist Friedrich von Huene*, Reuning in 1925 described the as yet unknown fossil-bearing strata of the Karoo Supergroup between the Ugab and Huab Rivers in the southern Kaokoveld, including descriptions of Mesosaurus fossils at Doros (some 40 km north-west of the Brandberg) and of the now famous dinosaur tracks on the farm Otjihaenamaparero, near Kalkfeld in central Damaraland. That same year he joined Hans Merensky* in South Africa and went prospecting for platinum in the Bushveld Igneous Complex. They found platinum-bearing rocks in the Potgietersrus (now Mokopane) district and discovered an unusual contact-metamorphic deposit containing very large crystals of Sperrylite (platinum arsenide). Reuning published a detailed description of the relationship and chemistry of the various rock types in this region of the Bushveld Complex in the Neues Jahrbuch für Mineralogie (Stuttgart, 1927).
In 1926 Reuning, Merensky and Dr Izak Celliers* discovered the rich diamond deposits at Alexander Bay, just south of the Orange River mouth. Reuning, who was largely responsible for proving their richness and extent, described the discovery in the Mining and Industrial Magazine (1928) and elsewhere. In his search for the origin of the coastal diamonds he found many kimberlite pipes further inland, but these contained no diamonds. He described this work in a comprehensive paper, "Der Ursprung der Küstendiamanten in Süd- und Südwestafrika", in the Neues Jahrbuch für Mineralogie in 1931. His investigation of the distribution and origin of the coastal diamonds, carried out over a period of some 20 years, demonstrates his perseverance in tackling a difficult problem. Later work has supported his view of an inland origin of the diamonds, rather than a sub-marine origin as proposed by Merensky. Also in 1931 he published a detailed stratigraphic study of a coastal region south of the Orange River, "The Pomona quartzite and the oyster horizon on the west coast of Africa", in the Transactions of the Royal Society of South Africa.
Reunert was also interested in archaeology and ethnology and carried out the first archaeological excavation in the vicinity of the Erongo Mountains and Spitzkoppe in Namibia. He also published a paper on the varieties of microliths found on the farm Donkerhoek, west of Rehoboth, in Chemie Erde (1933). The wide range of subjects that he investigated demonstrate his versatility as a scientist.
At the Fifteenth International Geological Congress, held in South Africa in 1929, Reuning read two papers dealing with differentiation processes in the Karoo lavas, and with differentiation in South African volcanic rocks in general. Both were published (in German) in the Comptes Rendus of the Congress (1930). He returned to Germany in 1929 and was appointed associate professor of mineralogy and petrography at the University of Giessen. During his tenure there he published further papers on the geology of the north-western Cape: "A contribution to the geology and palaeontology of the western edge of the Bushmanland plateau..." (Transactions of the Royal Society of South Africa, 1931, Vol. 19, pp. 215-232) and "The composition of the deeper sediments of the pipe at Banke, Namaqualand, and their relation to kimberlite" (Ibid, 1934, Vol. 21, pp. 33-40). In 1935 he resigned his post, settled in Cape Town as a consulting geologist, and described "Die goldfelder von Ondundu in Südwestafrika" (Geologische Rundschau, 1937). As a German citizen he was interned by the South African government during World War II (1939-1945). His collections of minerals and archaeological remains, as well as his diaries, which had been presented to scientific institutes in Berlin and Frankfurt, were destroyed during the war.
Reuning remained professionally active to 1957, when he undertook his last field work with Dr Henno Martin. Together they described the pre-Karoo land surface, Karoo sediments and Karoo volcanics of the southern Kaokoveld in the Neues Jahrbuch für Mineralogie, Geologie und Pälaeontologie (1957). During the last few years of his life Reuning grew vines and olive trees on the small farm Vlottenberg, near Stellenbosch. He became a member of the Geological Society of South Africa in 1910.