John R. le B. Tomlin, eldest son of John Read Tomlin and his wife Sarah Elizabeth Lund, was educated at Arnold House School, Chester, and at Winchester College in Hampshire. In Winchester he took an active part in the College Natural History Society and published his first paper in their magazine in 1882. In 1883 he won a scholarship to Pembroke College, University of Cambridge, where he studied classics and Sanskrit, qualifying as Bachelor of Arts (BA) in 1886. Years later he was awarded the degree Master of Arts (MA). While at Cambridge he became seriously interested in natural history, especially in molluscs and began to form his magnificent collection of shells. While coaching pupils in Liverpool he also studied insects and formed an outstanding collection of British beetles.
In 1890 Tomlin became senior assistant master at Llandaff Cathedral School in Cardiff, Wales, and from 1899 to 1902 taught at Stancliffe Hall School, Darleydale, Derbyshire. After inheriting some money he was able to spend more time on expanding his collections and meeting other natural historians. In 1906 he married Eleanor Marjorie Kensington, who assisted him in his collection activities. In 1916 the British Museum (Natural History) asked him to assist in the Department of Molluscs, which he did until 1948. Meanwhile he continued to expand his own mollusc collection by purchasing the collections of others. A few years before his death he presented both his mollusc collection and his collection of 82 000 beetles to the National Museum of Wales at Cardiff.
Tomlin published extensively on both beetles and molluscs, including 'Zoological results of the Cambridge Expedition to the Suez Canal, 1924: Report on the Mollusca' (Transaction of the Zoological Society of London, 1927), Hertfordshire Coleoptera (1949-1950, in 2 parts) and The Mollusca of Macquarie Island (1948), the latter collected by the British, Australian and New Zealand Antarctic Research Expedition of 1929-1931. Many of his papers dealt with South African Molluscs, including 'Description of a new species of Tenagodtis from South Africa' (Journal of Molluscan Studies, 1918); a series of 'Reports on the marine Mollusca in the collections of the South African Museum' (Annals of the South African Museum, 1925, 1927, 1928, 1931, 1932, 1934, 1937 and 1948); and 'On South African marine Mollusca, with descriptions of new species' (Annals of the Natal Museum, 1926 and 1931).
Tomlin was a founder member of the Malacological Society of London and served as its president from 1916 to 1919. In 1886 he became a life member of the Conchological Society, of which he was president for 1920/1 and again for 1948/9. He was a life member also of the Belgian Societe Royale Zoologique and of the Societe Linnienne de Lyon, and a Fellow of the Entomological Society. For more than 40 years he was the editor of the Journal of Conchology.