Robert T. Pett spent his entire career as an assistant at the Royal Observatory, Cape of Good Hope. He was initially appointed as third assistant in June 1876, arriving for duty (presumably from England) on 1 August that year. In August 1884 he was promoted to second assistant, and in September 1900 to assistant. He remained at the observatory to 1919.
In 1882 Dr David Gill*, director of the observatory, sent Pett to Durban to take temporary charge of the Natal Observatory, which was in the process of being established there. He arrived around the middle of June. The director of the Natal Observatory, E.N. Nevill*, arrived in Durban on 27 November that year, but by this time Pett had probably already left, for on 6 December 1882 he assisted W.H. Finlay* in observing the transit of Venus at a temporary observatory near Aberdeen Road. Among others Pett appears to have made many of the observations on which S.S. Hough* later based his "Ephemerides of 99 circumpolar stars, derived from observations 1830-1917", published in the Annals of the Cape Observatory in 1922. According to the General directory and guide book to the Cape of Good Hope... (1885, 1886) Pett was a Fellow of the Royal Astronomical Society (FRAS).