Mylne was born in
Great Amwell,
Hertfordshire, the son of
William Chadwell Mylne (1781–1863) and grandson of
Robert Mylne (1733–1811), both of whom were chief engineers of the
New River Company. His mother was Mary Smith Coxhead (1791–1884). Robert William himself also practiced as an architect and engineer. In 1836 he worked on designing new docks at
Sunderland, and he traveled to Italy and Sicily in the early 1840s. He acted as his father's assistant for twenty years, becoming an authority on water supply. He was later appointed Engineer to the
Limerick Water Company, and provided a fresh-water supply for one of the
Spithead forts. He wrote a number of books, primarily on the subject of the
geology of London, as well as producing several
geological maps of the area. His 1856 "Map of the Geology and Contours of London and its Environs", produced at a scale of 4 miles to 1 inch, was the first comprehensive geological map of London. In 1859 he was one of a party of eminent scientists, including
Joseph Prestwich,
Sir John Evans,
Hugh Falconer, Godwin-Austen and John W. Flower, who investigated the discovery of ancient human remains in the
Somme valley, establishing the
antiquity of man. He was an associate of the
Institute of British Architects from 1839, and a member of the
Geological Society from 1848 and of the
Smeatonian Society of Civil Engineers. He died at Home Lodge in Great Amwell in 1890. He is buried in the Mylne family vault at St John the Baptist Church in Amwell, designed by his ancestor
Robert Mylne. ==Family==