William Mann (1817–1873) was an English astronomer active at Royal Observatory, Cape of Good Hope, where he erected a new transit-circle in 1855 and made valuable observations.
In 1837 Admiral Shirreff procured him the post of second assistant at the Royal Observatory, Cape of Good Hope, and after due preparation he entered upon his duties in October 1839.
His health, impaired by hardships, was recruited by a trip to England in 1846, and on his return in December 1847 he engaged, as first assistant, in the ordinary work of the observatory.
His observations of the great comet of December 1844, and of the transit of Mercury on 4 November 1868, were communicated to the Royal Astronomical Society, of which body he was elected a member on 10 March 1871.
From a chest disorder, contracted through assiduity in cometary observations, he sought relief at Natal in 1866, in England in 1867, but was attacked in 1870 with debilitating effect by scarlet fever, of which two of his children had just died.