David Arnot received his education at the South African College, Cape Town, from 1836 to 1838 and thereafter settled in Colesberg as a general agent. In 1860 he was appointed agent of the Griqua chief Nikolaas Waterboer. Motivated by self-interest and British-imperialist tendencies he devoted his energies to promoting the interests of the Griquas of Griqualand West and opposing those of the settlers in the Orange Free State, particularly in obtaining for the Griquas possession of the territory south of the Vaal River and west of the Ramah-Platberg line. Largely through his efforts Griqualand West (including the diamond fields) was proclaimed British territory in 1870. Arnot made a number of land claims in the territory and was eventually granted the farm Eskdale and another block of farms near the confluence of the Orange and Vaal rivers. In 1874 he became a member of the Legislative Council of Griqualand West. He has been described as an emotional, ostentatious, unscrupulous, and highly intelligent man. With his friend F.H.S. Orpen*, Surveyor-General of Griqualand West, as co-author, he published a book entitled The land question of Griqualand West: an inquiry into the various claims of land in that territory; together with a brief history of the Griqua nation (Cape Town, 1875). He settled on his farm Eskdale in the eighteen-seventies, but moved to Cape Town in 1880.
Arnot was a naturalist with wide interests. During his stay in Colesberg he collected plants, particularly succulents, and before or in 1859 sent a box of living succulent plants to the Royal Botanical Gardens at Kew, London, for which W.H. Harvey* thanked him in the preface to Volume 1 of the Flora Capensis (1860). His contribution included living aloes, stapelias, crassulas, mesembryanthemums, and cotyledons. Later he also contributed some Stapeliads to Sir Henry Barkly's* collection. In due course the species Stapelia arnotii, Talinum arnotii, Haemanthus arnotii, and Hypoxis arnotii were named in his honour, as was the land snail Sheldonia arnotti. He had an interest also in ornithology and Arnot's Chat, Myrmecocichla arnoti, still carries his name. Other material that he collected included fossil reptiles and mammals. He was elected a Fellow of the Linnaean Society and of the (British) Geographical Society in 1875 and became a corresponding member of the South African Philosophical Society in October 1877.