Hugh Robert Mill, Scottish geographer and meteorologist, was the son of Dr James Mill, medical practitioner and farmer, and his wife Harriet Gordon Sage Davidson. He studied chemistry and physics at the University of Edinburgh and was awarded the degree Bachelor of Science (BSc) in 1883. The next year he worked on a fellowship at the Scottish Marine Station at Granton, studying mainly the chemistry and temperatures of coastal waters. In due course he developed an interest also in meteorology, oceanography, geography and polar studies. He was awarded the degree Doctor of Science (DSc) by the University of Edinburgh in 1886. From 1887 to 1892 he was a lecturer in geography and physiography (the science of the surface of the earth and the interrelation of air, water and land) at Heriot-Watt College, Edinburgh. He then moved to London and became the librarian of the Royal Geographical Society until 1900, publishing his Catalogue of the Library of the Royal Geographical Society in 1895. Meanwhile he had also been an extension lecturer (providing extramural instruction) for the Universities of Edinburgh and St Andrews from 1887 to 1900. In 1901 he was appointed joint director (from 1903 sole director) of the British Rainfall Organisation, a post he held until 1910. From 1903 to his retirement in 1919 he also served as rainfall expert to the Metropolitan Water Board (London).
Mill was a Fellow of the Royal Society of Edinburgh (elected February 1885) and of the Royal Geographical Society. He served as president of the latter in 1907-1908 and as vice-president during 1927-1932, and received its Victoria medal in 1915. The University of St Andrews awarded him an honorary Doctor of Laws (LLD) degree in 1900.
Mill became a member of the British Association for the Advancement of Science in 1885, served as president of its Section E (Geography) in 1901, and was a member of several of its committees. In 1905 he attended the joint meeting of the British and South African Associations for the Advancement of Science in South Africa. At the meeting in Cape Town on 18 August he read a paper in which he compared the long-period rainfall records of the Royal Observatory, Cape of Good Hope, with those of the Royal Observatory at Greenwich (London), for the period 1841 to 1904. The comparison indicated that although one wet spell (1858-1862) and one dry spell (1894-1901) affected both places, in most cases a wet spell at the Cape corresponded to a dry spell in London, and vice versa. Upon his return to the United Kingdom he published an account of the places he had visited in southern Africa in a paper entitled "South Africa as seen by a meteorologist during the visit of the British Association in 1905", which was published in the Quarterly Journal of the Meteorological Society in 1906. He also contributed a section on the rainfall of the Cape Peninsula to a report by H. Rofe, Cape Peninsula water supply; report presented to the joint water committee (1905).
Mill had a long standing interest in Antarctica. He inspired several polar explorers and published The siege of the South Pole (London, 1905). Many years later, in 1923, he wrote a biography of the polar explorer Ernest Shackleton. His collection of more than 500 works on Antarctica was presented to the Scott Polar Research Institute. He was for some time the editor of the Meteorological Magazine and the editor of a very comprehensive work, The international geography (New York, 1898, 1088 pp), of which several later editions appeared. His other publications included a book on The realm of nature; an outline of physiography (London, 1892) and many papers on meteorology, the temperature and salinity of British coastal waters, physical geography, antarctic exploration, surveys of English lakes, and other topics. In 1919 he had to abandon his rainfall work because of failing eyesight, but by that time had collected the available rainfall records for the British Isles back to 1677.
In 1889 Mill married Frances McDonald. After her death in 1929 he married Alfreda Dransfield in 1937.