The Royal Observatory was founded at the Cape of Good Hope in 1820 following a proposal by the Commissioners of the Board of Longitude, who included John Pond, the Astronomer Royal, and Sir Joseph Banks, President of the Royal Society. The main task of the new observatory was to extend the work of the Northern Observatories and to cooperate with them in the determination of fundamental astronomical data. Fearon Fallows was appointed 'His Majesty's Astronomer at the Cape of Good Hope' on 26 October 1820, and he arrived at the Cape with his wife the following year.

Falllows's first task was to select a site for the new observatory. The location had to meet three conditions - to be in sight of Table Bay, so that visual time signals could be passed to ships anchored there; to be far enough east of Table Mountain to allow an unobstructed meridian; and to be on government-owned land. Fallows was hindered in his efforts by the nature of the terrain, much of which was totally unsuitable for building. He eventually selected Slangkop, a rocky hill three miles east of Cape Town.

Fallows was inexperienced in practical astronomy, but gathered as much advice as possible before he left England. He began observing bright stars before the construction of the Observatory was completed by using small semi-portable instruments. His catalogue of 273 stars was published by the Royal Society in 1824, and constituted the first formal publication of the Cape Observatory.

The Observatory buildings were completed in June 1828. At this point the Observatory was a mere block of masonry on a exposed rocky hill, lacking roads, an adequate water supply and stabling or outdoor accommodation of any kind. Fallows applied to the Admiralty for a small grant of money for planting trees to provide protection from wind and dust, but was informed that he must fund the improvements at his own expense. Mr Skirrow, the Admiralty Clerk of Works, was unable to give his full attention to the completion of the Observatory, and it was not until the end of 1828 that the piers for the instruments were erected and the main building was completed. A transit circle, mural circle and Hardy Clock were also installed. Fallows devoted himself initially to working with the transit instrument. He was hindered in his efforts by the poor quality of his assistants, and his problems were compounded by a scarlet fever epidemic in 1830. Weakened by illness and stress, he died on 25 July 1831, at the age of 43.

Fallows was succeeded by Thomas Henderson (1798-1844). Henderson had gained experience in observation through using the telescopes at Edinburgh. When he arrived at the Cape in May 1832, he was already well versed in practical astronomy and was master of the associated theory and in methods of reduction. During his short time at the Cape, he accumulated a large number of accurate observations, from which he was able to draw up a catalogue of positions for 174 of the brightest southern stars. Henderson's most noteworthy work was his deduction of the distance of Alpha Centauri from the Earth using the parallactic displacement of the routine mural circle observations, although he did not receive the credit for this contribution at the time. He failed to settle at the Cape, and resigned in May 1833, returning to Edinburgh.

The new Astronomer Royal was Thomas Maclear (1794-1879), who arrived at the Cape on 5 January 1834. One of his first steps was to set up a routine of observing, although throughout his term of office the volume of observations greatly exceeded the limited facilities at his disposal for their reduction. Several new instruments were acquired. The most notable was the Airy Transit Circle, which was installed in 1855, replacing the early transit and mural circles, and which remained important to the Observatory's work until its dismantling in 1950. Maclear received support and encouragement from Sir John Herschel (1792-1871), who arrived at Cape at the same time as him with plans to set up his own private observatory.

Maclear gave much attention to land surveys and magnetic, meteorological and tidal observations. From 1838 to 1847 he spent a considerable amount of time organising and participating in the co-measurement and extension of the meridian arc measured by Abbe de Lacaille, carrying out observations and ironing out discrepancies. He also introduced the Meteorological Commission and the Commission of Standards for Weights and Measures, and assisted in the establishment of lighthouses. He was responsible for instructing the explorer David Livingstone in the use of the sextant and gave much time to the reduction of Livingstone's observations. One of the major events of this period was the fire in the Magnetic Observatory on 12 March 1852, which led to its complete destruction and the loss of all the papers and documents. No explanation was ever found for the fire, which started in the middle of the night.

Maclear retired in 1870 and was succeeded by Edward James Stone (1831-1897). Stone had been Chief Assistant to G.B. Airy for ten years at the Royal Observatory, Greenwich. His main work at the Cape was the reduction of Maclear's observations and the systematic observation of the 12,000 stars brighter than the seventh magnitude in the southern sky. He studied and observed the total solar eclipse of 16 April 1874, and confirmed Young's spectroscopic discovery of the 'reversing layer' above the bright surface of the Sun. He also gave much thought to the way in which the speed of sound could be measured, and found it by measuring the interval between the flash of the Cape Town noon gun and the instant the sound was heard at the Observatory. Stone was elected Radcliffe Observer at Oxford in 1878 and left to take up that position the following year.

The next Astronomer Royal was David Gill (1843-1914). Gill spent his early years at the Cape wiping out the arrears of reductions and reconditioning existing instruments, including the Airy Transit Circle. During his tenure he developed the Observatory's collection of instruments. A Repsold 7-inch heliometer was installed, and was used to determine the solar parallax of the minor planets Iris, Victoria and Sappho in co-operation with a number of northern observatories. The Victoria Telescope was also added, in 1901, and was employed in the first major programme to determine solar parallax from the radial velocities of a selection of stars round the ecliptic. A third major addition was a new reversible transit circle, which was designed by Gill himself and constructed by Troughton and Simms.

One of the most important developments during this time was the compilation of the 'Carte du Ciel' in cooperation with astronomers at Paris. The aim of the project was to prepare a photographic chart for the whole of the heavens showing stars down to the fourteenth magnitude, together with a catalogue giving precise positions for all stars down to the eleventh magnitude, which amounted to over a million stars. The Cape Observatory undertook a large part of this work. A suitable telescope, the Astrographic Refractor, was acquired, which was ready for use by 1892, and a complete set of plates of the Cape Zone was completed in 1896. However, the twelve volume catalogue was not published until 1926, and appeared in an incomplete form.

A second project brought more immediate success. Photographs taken of the bright comet of 1882 drew Gill's attention to the possibility of charting and measuring star positions by means of photography. His ideas on this subject culminated in the production of the Cape Photographic Durchmusterung - the first star catalogue produced from photographic measurements of the sky, which gave the approximate positions and brightness of nearly half a million southern stars.

While in South Africa, Gill was active in a number of other fields. The most notable of these were the surveys of Africa. He acted as the head of surveys of Southern Africa and arranged various geodetic and boundary surveys and the determination of the latitude and longitude of important ports.

Gill retired in 1906 and was replaced with his assistant, Sydney Samuel Hough (1870-1923). Hough's principal task on coming to office was to ensure that Gill's programmes were brought to a conclusion. His major contribution in this regard was the completion of a long series of measures for the Cape Zone of the Astrographic Catalogue, the results of which were published. These measures were used in combination with meridian observations to give highly accurate positions in spherical coordinates for over 20,000 of the brightest stars in the Cape Zone. In later years Hough suffered from stress as a result of the pressures of the First World War and the disruption caused by a severe 'flu' epidemic in 1918 that affected many members of staff. He died in 1923 and was replaced by Harold Spencer Jones (1890-1960). Spencer Jones was the Chief Assistant at Greenwich when he received his appointment as Cape Astronomer. He arrived at the Cape at a time when many changes were required - Gill's programmes were nearing completion, many of the staff were due to retire and much of the equipment was in need of repair. Spencer Jones had the Observatory modernised and built a new office block. As staff retired, he replaced them with young, enthusiastic South Africans. The Astrographic Zone was completely re-photographed. This allowed the new plates to be compared with those of 25 years earlier and the proper motions be derived. Just over 40,000 stars were ultimately measured in this way. Work was also carried out on the Eros project of 1930-1931 to determine the solar parallax of Eros, and on the preparation of the Cape Photographic Catalogue for 1950.0 (C.P.C. 50), which gave precise positions, magnitudes, colours and proper motions for nearly 70,000 stars distributed over the southern sky. Spencer Jones left the Cape in 1933 to become Astronomer Royal at Greenwich.

The next holder of the post was John Jackson (1887-1958). Jackson continued much of the work that had been started by his predecessor. His term of office was marked by a focus on the main observatory programme, which led to the accomplishment of an immense amount of routine work. Jackson concentrated on the parallax programme and oversaw an improvement in the quality of individual determinations. The work for the Astrographic Zone project was also completed. The Observatory continued to function properly during this period despite losing many of its staff to the demands of the Second World War.

On Jackson's retirement in 1950, he was succeeded by his Chief Assistant, Richard Hugh Stoy (1910-1994). Stoy was the last holder of his office to have the title 'Her Majesty's Astronomer at the Cape of Good Hope'. He initiated the photographic photometry programme to provide the magnitudes and colours of the 70,000 stars in the C.P.C. 50. During the 1950s and 1960s this developed into extreme precision photometry programmes. He was also responsible for renovations to equipment. The Gill Transit Circle and the Victoria Telescope were overhauled, and the Observatory received a 30-inch reflector from Dr W.H. Steavenson, which was used for infra-red photometry. The most significant addition was the Elizabeth 40-inch reflector, which was completed in 1963. The secondment of staff from the Royal Greenwich Observatory and the presence of numerous overseas visitors meant that the Observatory's equipment was used extensively.

The Observatory underwent major changes during the 1960s. Following the creation of the Science Research Council on 1 April 1965 as part of the reorganisation of scientific research in Britain, the Cape Observatory was incorporated into the Royal Greenwich Observatory. During this period the growing problems caused by the lights of Cape Town and the highways to the observation of faint objects prompted a search for a new site for the Observatory, with site testing beginning in 1967. The late 1960s saw the winding up of the Observatory's affairs. Its major programmes were reaching completion by 1968, and the last volumes of the C.P.C. 50 and Cape Annals were published that year. Stoy himself resigned during 1968.

George Harding was appointed Officer-in-Charge of the Observatory in January 1969, and oversaw its transfer to a new location at Sutherland. The new site was officially opened in March 1973, with Sir Richard Woolley (1906-1972) becoming the first Director of the South African Observatories. The Cape Observatory was retained as a base for the computation and analysis of results.