Robert Pinchin was articled to Mr W.J. Lord, a civil engineer formerly attached to the Royal Engineers, in 1842. After completing his apprenticeship he was employed by Joseph Bazalgette, who was surveying a proposed railway from Birmingham to Stratford-on-Avon. In 1846 he came to Cape Town in the hope of finding employment as a civil engineer, but on the advice of the surveyor-general of the Cape Colony, Charles D. Bell*, decided to concentrate on surveying. With Bell's support he passed the necessary government examination and was registered to practise as a land surveyor. He settled in Port Elizabeth, where the laying out of the town and its suburbs became his life's work. Among others he produced a plan of plots in Port Elizabeth (1852), a survey map of some land in the Uitenhage district (1856), a map of the Salt Pan lands near Port Elizabeth (1858), three plans of plots at Port Elizabeth for locating portions of the German Legion (1857), a street plan of part of Port Elizabeth (1860), plans of low water soundings in Port Elizabeth harbour (1867), the drift sands near Port Elizabeth (1871), and the tidal flow of the Zwartkops River (1882). About 1863 he was for a short time in partnership with George W. Smith*, but the partnership was dissolved as a result of a shortage of work. From 1879 he was a partner of Mr H.L. Spindler, former assistant to the chief resident railway engineer. Pinchin's survey work, with some civil engineering projects in between, continued until at least 1885 and proved quite lucrative. He was elected an associate member of the (British) Institution of Civil Engineers in February 1874.
Pinchin was at some time elected a Fellow of the Geological Society of London (FGS). His earliest known contribution to geology consisted of a geological section of Lion's Hill, Cape Town, which he showed and discussed at a meeting of the short-lived Port Elizabeth Natural History Society (1866-1867) on 22 November 1866. His claim that the (Malmesbury) slate, dipping westwards, underlies the Cape granite prompted Dr R.N. Rubidge* to participate in the discussion. In 1871 Pinchin wrote an article on "The bird islands: Geology of St Croix" for the Cape Monthly Magazine (Series 2, Vol. 2, pp. 354-357). However, his most important paper was "A short description of the geology of part of the Eastern Province of the Colony of the Cape of Good Hope", which was published in the Quarterly Journal of the Geological Society of London (1875, Vol. 31, pp. 106-107). In this paper he supported the view of Dr Rubidge that the so-called claystone-porphyry (later recognised as tillite, part of the Dwyka Formation) is a metamorphic rock. He also sketched a geological section through the colony from Port Elizabeth northwards. The original sketch was presented to Albany Museum, Grahamstown, by his former partner, George W. Smith, in 1909. At some time before 1883 "Mr Pinchin" - presumably him - presented numerous molluscs, mainly from Algoa Bay, to the Albany Museum.
In 1888 Pinchin and J.M. Leslie* were nominated by the Eastern Province Naturalists' Society (founded in 1882) to represent the society on the Port Elizabeth museum committee. Around that time Pinchin arranged the society's collection of geological specimens. However, he died soon afterwards after contracting pneumonia. During 1889 his geological journals were presented to the society's library. He was a reserved person, but was enthusiastic about professional matters.