John Templer Horne was the eldest son of Harry R. Horne and his wife Susan Blake. From November 1849 he was an assistant teacher in the government school at Wynberg, Cape Town. In March or April 1854 he was transferred to the office of the Surveyor-General of the Cape Colony as additional clerk, and remained there for the rest of his career. In 1857 he applied to be examined for registration as a surveyor and in due course was admitted as a sworn land surveyor. By 1867 he had been promoted to first clerk and that year was granted three months leave on full pay and six months on half pay, presumably to visit England. For some time he was a member of the Land Board. In December 1872 he was promoted to assistant surveyor-general, working under Abraham de Smidt* to 1889 and then under Leopold Marquard* for another three years. In July 1892 he succeeded Marquard as surveyor-general of the Cape Colony and retained the post until his retirement in 1902. He was succeeded by C.H.L. Max Jurisch*.
During Horne's time as surveyor-general his chief map compiler was C. Neumann Thomas*. When a new map of the Colony had been completed [perhaps the 1:800 000 map of 1896] the story goes that a dispute arose over whose name should appear on it. Although Thomas had done most of the work, the map was published under Horne's name. His name also appears on several maps of Pondoland, showing the location of tribal chiefs in the districts of Ngqeleni, Port St Johns, Lusikisiki, Flagstaff, and Tabankulu (1894); a map of the Cape Colony and neighbouring territories on which all public roads were indicated (3 sheets, 1895); and a hydrographical map of the Cape Colony and neighbouring territories (1895).
Several other members of the Horne family became surveyors. William Nevill and George Remington Horne, both land surveyors in the Cape Colony, were sons of J. Templer's brother William Henry. George Remington's son, John Remington (Jack) Horne, also became a surveyor. Horne was married to Antoinette Cheltenham.