John Grant was appointed as an astronomical computer at the Natal Observatory by the government astronomer of Natal, Edmund N. Nevill*, in November 1883. The observatory had been established the previous year and he was Nevill's first assistant. In 1885, when Nevill obtained the help of several ladies to do computations, John became astronomical assistant. However, he had to give up his post at the end of 1886 because of a shortage of funds. Nevill was able to re-appoint him in February 1888 and he remained astronomical assistant until 1890, when he was succeeded by Mabel Grant (See Mrs Mabel Nevill*), who was probably his sister.
Several other members of the Grant family, in addition to John and Mabel, helped out at the observatory for a number of years: Miss B. Grant worked there as computer in the mornings from 1887 (probably from 1885) to 1891, and then as meteorological assistant to 1894. Miss N. Grant was a part-time computer during 1890-1891, Miss A. Grant during 1891-1892, Miss I. Grant during 1892-1893, and Miss J. Grant during 1893-1894.