R T Pentreath was born in Mousehole, near Penzance, in August 1806, the son of Richard Pentreath , a schoolmaster and Julia nee Badcock.

He won the Silver Medal (lst prize) in the 1835 EXHIBITION of the newly 'crowned' Royal Cornwall Polytechnic Society (RCPS) of Falmouth. At that stage he was a servant at Clowance, one of the family seats of the St Aubyns (of St Michael's Mount), and later an attendant on Sir R Vyvyan of Trelowarren with whom he travelled on the continent, recording the places visited.

His painting at the RCPS was Newlyn, The Pilchard Factory, and was fulsomely described as a very beautiful picture attracting general admiration, and affording an admirable specimen of native talent. 'The houses and the pier of Newlyn, with the brig lying beside it, all partook of that harmonious colouring, which constituted one principal excellence of this picture.' In 1846 he took the bronze medal (Second prize) for his painting of Pilchard Tucking in oils. He became a popular portrait and landscape painter, exhibiting regularly at the RA (1853-1868). According to Holmes many of his works are unsigned and attributable only by the engravings produced. Some of his work, due to the subject matter and being unsigned have been attributed to Thomas Hart. Many of his works were engraved by Vibert & Tonkin and Besley at Exeter.

Aside from his painting, Pentreath accepted commissions for map-making.  He is believed to have been responsible in 1841 for the 'Plan of the Tenement of Bossigran (sic) in the parish of Zennor' then the property of H C Phillips Esq.  This very large map, 34 inches x 51 1/2 inches, was gifted to the Hypatia Trust in 2008 and is now lodged permanently with the Cornwall Records Office (CRO). 

In 1841 he was living with his wife, Mary Ann and two sons at Clarence Street, Madron and is described as a Spirit Merchant. However, ten years later while still living at the same address he is then described as an artist. By 1856 he was living at Exmouth in Devon while in 1861 he and his wife are found as one of three families living at 30 Moore Street, Chelsea, London. Whether this was a short term visit or longer is not known for he died in Exmouth in January 1869 at the age of 62.