Mabel Grant was the daughter of William Grant, merchant, and his wife Sarah, born Pilcher. She was the first South African women's singles tennis champion in 1891 and retained her title until 1894. From 1887 (and probably from 1885) she and several other ladies assisted Edmund N. Nevill*, government astronomer of Natal, as computers at the Natal Observatory during the mornings. Their work involved astronomical calculations, processing of meteorological observations, and the reduction of tidal observations. In 1890 Mabel became meteorological assistant at the observatory, and on 1 September the next year succeeded John Grant* (who may have been her brother) as (senior) astronomical assistant. She had been trained for the work by Nevill. They were married in 1894 and eventually had two sons and a daughter.
As Mrs Nevill, Mabel continued her work as senior astronomical assistant. The duties attached to the post were described by the government astronomer as follows: "The senior assistant maintains a general supervision over the whole of the details of the work of the Observatory; carries on the general and astronomical correspondence; assists when required in all observations made between eleven o'clock at night and eight o'clock the next morning; looks after, makes out, checks, and pays all accounts, keeping the necessary books, and preparing and rendering all the returns required by the Government; calculates all the ephererides, working lists, and tabular places; constructs all auxiliary tables and charts; reduces the magnetic observations and tidal records, and generally takes over the duties of the Junior and Meteorological Assistants when they are absent on leave or from illness" (Report of the Government Astronomer, 1903, p. 3). She resigned on 1 July 1902, but continued her work until her successor, R.F. Rendell*, arrived in April 1903.