Harold Hartley was born in London on 3 September 1878, only son of Harold Thomas Hartley and his wife, Katie. He was educated firstly at Mortimer College and then spent three years at Dulwich College, before going to Balliol College, University of Oxford in 1897. He graduated in 1900 with first-class honours in natural science (chemistry and mineralogy).

Hartley remained at Balliol, and in the year after his graduation became Natural Science Tutor and Bedford Lecturer in Physical Chemistry, a position which he held until 1931. He was also made Jowett Fellow of Balliol (1928-1931) and then a Research Fellow (1931-1941). In 1906 Hartley married Gertrude, eldest daughter of A. L. Smith, later Master of Balliol; they had one son and one daughter.

When the First World War broke out, Hartley served initially in the 7th Leicestershire regiment (1914-15), but as part of the attempt to counter German gas attacks was appointed as Chemical Adviser to the 3rd Army (1915-17). In 1917, newly promoted to lieutenant-colonel he was made Assistant Director Gas Services, at General Headquarters in France, and at the end of the war, he was made brigadier-general and Controller of the Chemical Warfare Department at the Ministry of Munitions (1918-1919). For his services, Hartley was awarded the Military Cross and was mentioned in despatches three times.

In 1919, Hartley served as a member of the Holland committee on the future of chemical weapons and continued as the senior scientific adviser to the government on these issues during the 1920's and 30's, serving on the chemical warfare board until 1950. He had returned to academic life after the war, combining teaching physical chemistry at Balliol with his position as director of research at the Gas Light and Coke Company (later the North Thames Gas Board), where he was to remain on the board until 1945, serving as deputy governor during the Second World War. In his own research, he concentrated on studying the electrical conductivity of solutions, work for which he was elected to the Royal Society in 1926. Hartley was also concerned in the development of chemical industry. In 1929 he joined the Fuel Research Board of the Department of Scientific and Industrial Research, which he later chaired (1932-47), and some years later served as Honorary Adviser on the development of home-produced fuels at the Ministry of Fuel and Power (1939-1947).

Hartley resigned his tutorial fellowship in 1930 to become vice-president and director of research of the London, Midland, and Scottish Railway Company, also chairing the new Railway Air Services (1934-45). Continuing his interest in air travel, he then became chairman of British European Airways (1946-47) and then of the British Overseas Airways Corporation (BOAC) (1947-49). He then became the first chairman of the Electricity Supply Council (1949-52), later acting as consultant to the Central Electricity Generating Board (CEGB).

Hartley retained his interest in science and the chemical engineering industry, with positions including: President of the British Association for the Advancement of Science (1950); President of the Institution of Chemical Engineers (1951-52 and 1954-55) and of the Society of Instrument Technology (1957-61); scientific adviser to the construction firm John Brown (1954-61); Chairman of the British National Committee and International Executive Council, World Power Conference (1935-50) and the Fourth World Power Conference (1950); President of the World Power Conference (1950-56); Chairman of the Energy Commission, Organization for European Economic Co-operation (1955-56). Hartley also had a particular connection with the Goldsmiths’ Company, and was its Prime Warden (1941-42).

Hartley was knighted in 1928. His honours include a KCVO in 1944, GCVO in 1957, and CH in 1967. His publications include: "Balliol Men" (1963); editing "The Royal Society: its origins and founders" (1960); "Man and Nature" (Romanes Lecture 1964); "Humphrey Davy" (1966); "Studies in the History of Chemistry" (1971). He died on 9 September 1972.