Henry Edward O'Neill, who had the rank of lieutenant in the Royal Navy, was British consul in Mozambique from 1879 to 1889 and was a Fellow of both the Royal Astronomical Society and the Royal Geographic Society. He completed thirteen exploratory journeys in northern Mozambique, including the first exploration of the Makua and Lomwe countries between Mozambique Island and Lake Malawi. His observations represent a large body of information on the peoples of northern Mozambique, on the history of the slave trade in the western Indian Ocean and on the expansion of Portuguese rule and the resistance to it by powerful local communities. His travels were described in some twenty papers published during 1880-1889 in the Proceedings of the Royal Geographical Society and other journals. For example, "A three months' journey in the Makua and Lomwe coountries" (1882, with map) described a journey during which he penetrated some 300 km inland towards Lake Malawi, with an account of the country and its inhabitants. "On the coast-land and some rivers and ports of Mozambique" (1882) contained information gained during six coastal voyages during which all noteworthy places between Cape Delgado (c. 11oS) and Quillimane (c. 18oS) were visited with a view to enquire into slavery and trade. His "Journey from Mozambique to Lakes Shirwa [now L. Chilwa] and Amaramba [now L. Chiuta]" (1884, with map) solved the question of the origin of the Lugenda River. "Eastern Africa between the Zambesi and Rovuma Rivers" (1885), a review of geographical discovery and the natural resources of Mozambique, included a description of the Zimbabwe ruins, which he regarded as of Phoenician origin. Two contributions on astronomical observations made during his journeys in northern Mozambique were published in the Scottish Geographical Magazine in 1885 and 1889 respectively. In another paper in the same journal, "Cyclone in Mozambique Channel" (1887), he discussed the theories of cyclone forecast and tested them against his own observations.
In December 1886 O'Neill set out on a journey from Delagoa Bay (now Baia de Maputo), via the lowest ford of the Tembe River (Rio Tembe), through Swaziland into the Transvaal, returning via Matala Poort in the Lebombo Mountains in January 1887. His report to the Foreign Office on this partly successful venture was published under the title "Journeys in the District of Delagoa Bay, December 1886 to January 1887" in the Proceedings of the Royal Geographical Society (1887).