William Mann, astronomer, was born in the Borough of Lewisham, now part of London, as the third son of Major-General Cornelius Mann. He was privately educated. The family moved to Gibraltar in 1830, where General Mann commanded the Royal Engineers. In 1837, having completed his schooling, William returned to England to study and prepare for a profession. Among others he took an engineering and surveying course at Chatham, Kent, where there was an engineering school. In April 1839 he was appointed second assistant to Thomas Maclear* at the Royal Observatory, Cape of Good Hope, and arrived at the Cape on 22 October, bringing with him standards of length, compensation bars and other survey instruments for Maclear's survey of an arc of meridian. Despite being colour blind Mann was artistically inclined and, although no extensive sketchbooks by him are extant, his sketches in letters, diaries and field survey books show him to have been an accomplished artist. He became firm friends with the observatory's first assistant, Charles P. Smyth*
Both Mann and Smyth spent much of their time during the next six years on the arc of meridian survey, first assisting Maclear in measuring, with great precision, a 13 km base-line in the Swartland and thereafter carrying out the triangulation. The angular measurements were made with the best available theodolites and were often repeated 100 times or more. The arc stretched from the Royal Observatory in Cape Town to a point beyond the Kamiesberg in Bushmanland (a total north-south distance of some 500 km), while the triangulation was extended eastwards from Cape Town to Cape Agulhas. It was difficult work, often carried out in poor weather conditions, and entailed considerable hardships. Mann performed his tasks with great skill and enthusiasm, preferring life in the field to routine work at the observatory. In November 1845, while engaged on the triangulation to Cape Agulhas, he fell from his horse and suffered head injuries that plagued him for the rest of his life. Smyth had just resigned his post and Mann succeeded him as first assistant in January 1846. However, his injuries necessitated a visit to England to recuperate. He returned to the Cape in December 1847 to take up his new post. He showed great aptitude for mathematics and was extremely patient and persevering, and fastidiously scrupulous in his work. Maclear declared that "His powerful intellect, his unflinching integrity and industry enable me to trust him with confidence on all occasions... being certain that whatever is practicable he will accomplish, and that what he does will be sure to be well done" (William Mann, 1948, p. 45). In January 1854 Mann married Caroline Maclear, one of Thomas Maclear's daughters. They eventually had five sons and six daughters.
After completion of the survey Mann was engaged in regular observing and calculating at the observatory. In October 1852 Maclear sent him to England to study the mechanics and method of mounting of a large new transit circle for the Cape. After his return in December 1853 his mechanical ability enabled him to carry out much of the work involved in installing the instrument without skilled assistance. Later he used the instrument to determine the "Mean right ascensions and north polar distances of stars compared with comet I, 1864, at the Royal Observatory, Cape of Good Hope..." (Memoirs of the Royal Astronomical Society, 1866). Three years later he published "Observations of the transit of Mercury over the sun's disc on November 4th, 1868, at the Royal Observatory, Cape of Good Hope" in the Monthly Notices of the Astronomical Society.
During Maclear's absence in England in 1859 Mann was in charge of the observatory for nine months. The next year he was appointed a member of the newly created Meteorological Committee (later Meteorological Commission) of the Cape of Good Hope, remaining a member to his death. In 1863 he was offered the post of government astronomer in Sydney, Australia, but declined it, perhaps hoping to succeed Maclear. By this time he had developed an interest in comets and among others made a long series of observations of Encke's comet. While involved in this work he began to suffer from a severe bronchial condition from which he never completely recovered. It led him to visit Dr Robert James Mann* in Pietermaritzburg during March and April 1866, and to pay an extended visit to England from May to November the next year. Though Maclear wished Mann to succeed him as Her Majesty's Astronomer at the Cape, he was considered too old, and perhaps too unwell. E.J. Stone* was appointed to the post when Maclear retired in 1870, with Mann staying on as first assistant. In March 1871 he was belatedly elected a Fellow of the Royal Astronomical Society. In June 1872 scarlet fever broke out at the observatory. Mann suffered a severe attack, while two of his children died from the disease. After 32 years of unspectacular but valuable service he retired in December 1872 as a result of ill health and died a few months later.