John Leslie Masson, son of Alexander Masson, a farmer, and his wife Jean (Jane) Leslie, qualified as a teacher at the University of Aberdeen. In 1871 he obtained an appointment as a teacher at Hilton College, near Pietermaritzburg, Natal. In December 1876 he married Dorothy Bowness, with whom he had two sons and a daughter. For some time after their marriage they lived in the Orange Free State (now the Free State) but returned to Natal before or in 1881. He was appointed acting clerk in the office of the surveyor-general of the Natal Colony on 15 December 1881, working first under P.C. Sutherland* and later under A.H. Hime. In 1882 he requested to be examined for a certificate of competency as a land surveyor, but did not qualify either at this time or later. From the beginning of 1889 he was promoted to chief clerk. On 1 July 1890 he became assistant [and acting] surveyor-general of the colony (succeeding Hime), until his appointment as surveyor-general was confirmed in January 1894. He remained in this post until his retirement in August 1908. From 1898 to 1901 he was a member of the Civil Service Board and after 1903 served on the board of trustees of the Natal Museum. He was awarded the Queens South Africa Medal for his services during the Anglo-Boer War (1899-1902).
Some topographic and cadastral surveying was carried out under Masson's direction, as well as some secondary triangulation during 1904. He supervised the preparation of a farm plan of the colony in 1902 at a scale of one mile to the inch (1:63 360). Around the same time he completed a topo-cadastral Map of the Colony of Natal on a scale of four miles to the inch (1:253 440), and named a farm in the Camperdown area after the surveyor August Hammar*. In 1906 he compiled a map of the Nkandla District. Early in March 1904 he attended a conference in Cape Town, attended by the surveyor-generals of all the British colonies in southern Africa, at which the further geodetic and topographical survey of southern Africa was planned.
Masson became a member of the South African Philosophical Society in 1899, and remained a member when it became the Royal Society of South Africa in 1908. By 1906 he was also a member of the South African Association for the Advancement of Science.