Sir Harold Spencer Jones was born at Kensington, London, on 29 March 1890. He was the third child, and eldest son, of Henry Charles Jones, an accountant with the Great Western Railway, and Sarah Ryland, who had been a schoolmistress. His father, who was interested in all branches of mathematics, encouraged his son's interest in this field, for which he demonstrated an early aptitude, and this ability was fostered by his teachers at the Latymer Upper School in Hammersmith. He won a scholarship to Jesus College, Cambridge, and in 1909 obtained First Class Honours in Part I of the Mathematical Tripos. He was classed a Wrangler in Part II of the Tripos for Physics in 1912, and that year was elected Isaac Newton Student. In 1913 he was second Smith's Prizeman and became a Fellow of his college.

Spencer Jones joined the Royal Observatory for the first time in 1913, becoming Chief Assistant in place of Sir Arthur Eddington. His first major task, in 1914, was to accompany C.R. Davidson to Russia to observe the solar eclipse, a trip which was cut short by the outbreak of the First World War. During the war, he served as Assistant Director for the inspection of optical supplies for the Ministry of Munitions. He was stationed at Woolwich, and made use of the nearby Royal Observatory and its few remaining astronomers to test thousands of pairs of binoculars sent in by the public for military use. After the war, he became interested in the variation of latitude, employing the Cookson Floating Telescope for his work on this subject and producing several papers. He also made a determination of the photographic magnitude scale of the North Polar sequence. In 1918 he married Gladys Mary Owens, with whom he went on to have two sons.

Spencer Jones' first period at Greenwich came to an end in 1923, when he was appointed to succeed S.S. Hough as H.M. Astronomer at the Cape of Good Hope. Despite operating within a tight budget, he managed to address many of the observatory's problems, such as the unsatisfactory living conditions, and was able to take on more local staff. One of his major tasks at the Cape was to sort through and arrange some twenty years' observations of the radial velocities of the brighter southern stars. This work resulted in the publication of a catalogue of radial velocities and the calculation of a number of orbits of spectroscopic binaries. He completed some of Sir David Gill's outstanding programmes and began some of his own, as well as studying the nova of 1925, known as Nova Pictoris. He also pursued an interest in positional astronomy, using the Victoria telescope at the Cape for a programme of photographic determinations of trigonometrical parallax. In 1924 he used Gill's 7-inch heliometer to observe Mars at its close approach to the Earth, and employed these observations and a series of photographic observations made with the astrographic telescope to derive a value for the solar parallax.

His major work during this period was the improvement of the value of the solar parallax using the minor planet Eros. A favourable opposition of Eros was to occur in 1930-1931, and the International Astronomical Union set up a Solar Parallax Commission (Commission 34) to organise its observation, with Spencer Jones as President. His observational programme for the project involved some twenty-four observatories worldwide, although the largest proportion of the work was carried out at the Cape. He took it upon himself to catalogue and evaluate all the results, which culminated in the establishment of a new value for the solar parallax. He was awarded the Gold Medal of the Royal Astronomical Society and the Royal Medal of the Royal Society for his work on this project.

Spencer Jones became the tenth Astronomer Royal in 1933, following the retirement of Sir Frank Dyson. One of his first tasks was to bring into operation two important new instruments: the 36-inch Yapp Reflector and the reversible transit circle, which replaced Airy's instrument. During his first years in office, he introduced major programmes, including work on latitude variation and the determination of time. The photographic zenith tube was introduced in place of the Cookson Floating Telescope for latitude variation work, and the new quartz oscillators replaced Dyson's Shortt free pendulum clocks as the standards of time. Quartz oscillators were also used for the determination of the variation in the rotation of the Earth, which led to Spencer Jones' noted paper 'The Rotation of the Earth and the secular accelerations of the Sun, Moon and planets' (1939).

The most significant decision made during this era was to move the Royal Observatory away from the pollution and light interference of London to a more suitable site. Spencer Jones first suggested this course of action in 1938, but it was not until after the Second World War that anything definite could be done. He personally searched much of Southern England for a suitable new home for the Observatory, eventually deciding on Herstmonceux Castle, near Hailsham, in Sussex. The castle was purchased by the Admiralty in 1946, and work began soon afterwards on the removal from Greenwich. The new observatory was given the name of the Royal Greenwich Observatory, Herstmonceux. Spencer Jones retired before the majority of the move had been completed.

Spencer Jones contributed to a number of other important developments at the Royal Observatory. He played a significant role in the introduction of the Isaac Newton Telescope (although he did not live to see the commencement of the construction work) through chairing the Royal Society Committee that decided on the need for a large telescope in Britain, persuading the Government to upgrade it from a 74-inch telescope to a 100-inch, and conducting the negotiations with the Treasury that led to the decision to finance the project. He was also responsible for setting up a workshop for the repair of Admiralty chronometers, which had previously been carried out away from the Observatory.

Spencer Jones held many important posts over the course of his career, including the presidencies of the International Astronomical Union (1945-1948), the Royal Astronomical Society (1937-1939) and the Institute of Navigation (1947-1949). His contributions to the horological industry in Britain were recognised when he was made President of the British Horological Institute and twice Master of the Worshipful Company of Clockmakers (1949 and 1954). He was also a Fellow of the Royal Society and an Honorary Fellow of Jesus College, Cambridge. He received many honours, including a knighthood in 1943, and K.B.E. in 1955. Other awards included the British Horological Institute Gold Medal (1948); the Janssen Medal of the Société Astronomique de France (1949); the Bruce Gold Medal of the Astronomical Society of the Pacific (1949); the Lorimer Gold Medal of the Astronomical Society of Edinburgh (1953); the Rittenhouse Medal of the Rittenhouse Astronomical Society, Philadelphia (1955); and the Joykissen Mookerjee Gold Medal of the Indian Association for Cultivation of Science (1957).

Spencer Jones retired in 1955, and devoted himself thereafter to the organisation of international science. He was heavily involved with bodies such as the International Council of Scientific Unions, which he represented at several U.N.E.S.CO. meetings. He died suddenly at his home in Kensington on 3 November 1960. He produced many published works over the course of his career, including the major works 'General Astronomy' (1934), 'World without End' (1935) and 'Life on Other Worlds' (1940).