sketcher, amateur photographer and surveyor, was born on 16 December 1819 at Lewisham, near London, son of Lieutenant-Colonel Cornelius Mann R.E., and Sarah, née Fyers. Educated at Gibraltar, then at the Royal Military College, Sandhurst, John abandoned a military career in 1838 in favour of surveying and was employed on the Trigonometrical Survey of Britain. He joined his elder brother, Gother Kerr Mann , at Sydney on 6 March 1842, arriving on board the Palestine . In October 1846 Mann was second-in-command on Ludwig Leichhardt 's second exploring expedition (aborted after eight months). The ink and pencil sketches he drew on the expedition, as well as views of Sydney in the 1840s and 1850s, are in the Dixson Library. A memoir, Eight Months with Dr. Leichhardt, in the years 1846-47 , published by Mann in 1888, revealed the extremely low opinion of Leichhardt held by many of those serving under him (including Mann). Mann’s lively ink sketch of Leichhardt in the bush with a bottle-tree in the background, however, though something of a caricature (as was, by all accounts, Leichhardt himself), is far more good-natured than the text.
Mann joined the Surveyor-General’s Department under Thomas Mitchell in March 1848. On 16 April 1857, at St Mark’s Church of England, Darling Point, he married Camilla Victoria, Mitchell’s third daughter. On 24 August 1858, Camilla’s younger sister, Blanche, wrote in her diary: 'Walked out to Milly’s… Watched John prepare his photographic apparatus, making the bath solution, which is composed of distilled water, nitrate of silver and alcohol … Thurs 6th September  ... Saw John taking views with his camera. At present they are not very good owing to defective collodion, but time will improve it much’. No surviving photographs are known.
Mann resigned from permanent employment as a NSW government surveyor after Camilla’s death in childbirth in 1863 but worked on contract until the late 1870s. He continued sketching well into old age. In 1884 he went to New Guinea for the proclamation of the Australian Protectorate as a representative of the Geographical Society of Australasia, reporting on the country in the society’s Proceedings (1889 and 1894). He also published occasional articles in the Sydney press.
Mann died on 7 September 1907 at his Neutral Bay home, Carthona (named after the Mitchell home at Darling Point), and was buried in St Thomas’s Anglican Cemetery, North Sydney. He had never remarried, raising his two sons and one daughter with the aid of an English nurse. All three children survived him. His architect and planner son, Gother Victor Fyers Mann (1863-1948), later became the Director of the Art Gallery of New South Wales. The Mann family papers are in the Mitchell Library.