Roland Trimen was initially educated privately, but from the age of 12 attended King's College School in London. His health was poor and in 1858 he was sent on a sea voyage to the Cape Colony. Although aged only 18 at the time, he knew enough about natural history to help E.L. Layard*, the curator of the South African Museum in Cape Town, arrange the museum's collection of beetles. At this time he also visited Knysna. He returned to the Cape Colony in 1860, joining the civil service in July that year as a third class clerk in the Auditor-General's office. In March 1862 he was transferred to the Colonial Secretary's office, and later to the office of the Governor (August 1873) and, as acting chief clerk, to that of the Commissioner of Lands and Public Works (January 1875).
From February 1866 to February 1867 Trimen stood in for E.L. Layard* as part-time curator of the South African Museum. In August 1872 he accompanied the Governor, Sir Henry Barkly, to Griqualand West as his acting private secretary and later that year was promoted to first class clerk. In January 1873, following Layard's departure, he was again appointed part-time curator, though he retained his clerical post and could devote only one day a week to the museum. In July 1875 he was made secretary to the local committee organising participation in the Philadelphia Centennial Exhibition of 1876. In July the next year he accompanied Premier J.C. Molteno on a mission to Britain. During his absence he was appointed full-time curator of the South African Museum (then a civil service post), assuming duty after his return from Britain in October. He held this position until 1895 and was the first resident insect taxonomist to be appointed in South Africa.
During the years before his appointment as full-time curator Trimen devoted his spare time to natural history, particularly to the study of Cape Lepidoptera. His first paper, "Entomology of the Cape of Good Hope", was published in the Transactions of the Entomological Society of London for 1858-1861, followed by "On some new species of South African butterflies" (Ibid, 1862-1863). He again helped to arrange the South African Museum's insect collection during 1864. In March-April 1867, during a visit to Natal, he collected specimens along the south coast and as far inland as Pietermaritzburg, and in 1870 visited the Albany district. However, he was not primarily a collector, devoting most of his energy to taxonomic research. In 1862 he published the first part of his Rhopalocera africae australis: A catalogue of South African butterflies, comprising descriptions of all the known species, with notices of their larvae, pupae, localities, habits, seasons of appearance, and geographical distribution. Part 2 followed in 1866. This work constituted the first attempt at a comprehensive account of South African butterflies and marked the beginning of the systematic study of insects at the museum. During the next three decades Trimen published a number of significant papers on Lepidoptera in British scientific journals, for example, "On the butterflies of Madagascar" (1864); "Notes on the butterflies of Mauritius" (1867); "Aspects of insect life in south-eastern Africa" (1867); "On some undescribed species of South African butterflies, including a new genus of Lycaenidae" (1868); "On some remarkable mimetic analogies among African butterflies" (1869); "Notes on butterflies collected by J.H. Bowker*, Esq., in Basutoland, South Africa, with descriptions of new species" (1870); "On some new species of butterflies discovered in extratropical southern Africa" (1873); "Some species of South African Lycaenidae" (1874); "On some hitherto undescribed butterflies inhabiting southern Africa" (1879); "Note on the capture of the paired sexes of Papilio cenea in Natal" (1881); "Some new species of Rhopalocera from southern Africa" (1881); descriptions of the butterflies collected in German South West Africa [now Namibia] by Axel W. Eriksson* (1891); and an account of the butterflies collected in south-east Africa by the hunter F.C. Selous* (1894). His most important work on this subject, however, was his monumental South African butterflies: A monograph of the extra-tropical species (London, 3 vols, 1887-1889), which was a greatly expanded version of his earlier book and in which he described some 380 species. In the preparation of this work he was assisted by J.H. Bowker* and in the preface he acknowledged the assistance of many local collectors. His publications made him the leading authority on South African butterflies of his time.
Trimen's studies of insects awakened his interest in the fertilisation of plants, a topic on which he corresponded with Charles Darwin*. He wrote two important botanical papers on the fertilisation of orchids: "On the fertilisation of Disa grandiflora L" (Journal of the Linnean Society (Botany), 1863) and "On the structure of Bonatea speciosa L with reference to its fertilisation" (Ibid, 1867). After his visit to Namaqualand in 1872 he published a note, "Vegetation of Little Namaqualand" in the London Journal of Botany (1873).
When he was being considered for the post of curator of the museum, Layard expressed the opinion that he was not a suitable candidate, as he lacked the versatility, wide scientific interests, and dedication to the museum that the post demanded. Events proved him wrong. Trimen was a successful curator and during his time in office the museum developed from an institution concerned mainly with display into a centre of research. He also widened his interests, publishing papers on a variety of zoological topics outside his field of specialisation. Some of these dealt with mimicry, protective coloration and imitation in various insects and arachnids, a subject in which he was far ahead of his time. Other examples are: "Note on the Colorado beetle" (Transactions of the South African Philosophical Society, 1877); "The so-called 'bonnet' of the southern right whale" (Ibid, 1878); three papers on birds: "A new species of roller (Coracias) from the Zambesi" and "An undescribed Laniarius from the interior of southern Africa" (Proceedings of the Zoological Society of London, 1880) and "On an apparently undescribed sun-bird from tropical south-western Africa (Cinnyris eriksoni, n.sp.)" (Ibid, 1882); "On a remarkable variety of the leopard, Felis pardus, obtained in the east of the Cape Colony (Ibid, 1883); "Note on the teeth of the ziphioid whale, Mesoplodon layardi" (Transactions of the South African Philosophical Society, 1886); "On the occurrence of a rare fish (Lophotes cepedianus) at the Cape of Good Hope" (Proceedings of the Zoological Society of London, 1891); and an extensive chapter on "The South African vertebrate fauna" in the Illustrated Official Handbook of the Cape and South Africa (1893), edited by J. Noble*.
Trimen was a member of the Vine Diseases Commission of 1880. In October 1881 he represented the Cape Colony at the international phylloxera congress at Bordeaux, France, and five years later became the first chairman of the Phylloxera Commission (1886) appointed by the Cape Government to study root rot in Cape vines. He attended the International Zoological Congress in Paris in 1887. In 1885 he married Henrietta B. Bull, but they had no children. For many years he was periodically in poor health and though his problems were probably at least partly psychosomatic he lacked the stamina to undertake much collecting. In 1895 he obtained six months leave for health reasons. He went to England and at the end of his leave resigned his position and was awarded a pension. During the previous years he had designed the present museum building, which was under construction when he left.
Trimen was a foundation member of the South African Philosophical Society, serving as its first secretary (1877/8), as a member of council until his retirement, and as president for 1883/4. His presidential address dealt with "Protective resemblances and 'mimicry' in animals". After his retirement he was elected an honorary member of the South African Philosophical Society and when this society became the Royal Society of South Africa in 1908 he was elected an honorary Fellow of the latter. He was a Fellow of the Royal Society of London (1882), the Linnean Society (1871), and the Zoological Society of London, and a member of the British Association for the Advancement of Science (1871) and the Entomological Society of London (1859), serving as president of the latter for 1897/8. In 1904 he was elected an honorary member of the South African Ornithologists' Unionl. In 1899 the University of Oxford awarded him an honorary Master of Arts (MA) degree. Many species of butterflies were named after him. His butterfly collections are housed in the South African Museum and in the British Museum (Natural History), London. He was a brother of Henry Trimen, editor of the Journal of Botany (London); the plant genus Trimenia was named after Henry, while the species Melianthus trimenianus was named after both brothers by J.D. Hooker* in recognition of their contributions to science. Other species named after him include the South African non-marine mollusc Natalina trimeni.