Johnson, Thomas (1863–1954), botanist, was born 27 February 1863 at Barton-upon-Humber, Lincolnshire, the fourth child in a family of four sons and a daughter of George Rex Johnson (1822–1893), owner of a tanyard in Retford, Nottinghamshire, and Mary Johnson (née Young, 1823–1909) of Rippingale, Lincolnshire. He was educated at Elmfield College, Heworth, York and subsequently at the Normal School of Science, South Kensington (from 1890 the Royal College of Science, and from 1907 part of Imperial College, London). The Normal School was then the premier British institution for innovative laboratory-based education in the sciences, pioneered by its dean, Thomas Henry Huxley.
As demonstrator in botany at the school from 1886 and from 1890 as professor of botany in the Royal College of Science for Ireland in Dublin, Johnson was in the vanguard of those (inspired by Huxley's educational reforms based on German exemplars) who spread the modern teaching of biology throughout Britain and Ireland. He was awarded a D.Sc. degree by the University of London in 1893. Initially his appointment as professor was part-time (owing to another commitment, to the Science and Art Museum in Dublin), but in 1899 he was appointed full-time professor of botany.
For nearly forty years in his post at the Royal College of Science, Johnson was a notable innovator in teaching, research, and administration of plant sciences in Ireland. In the early years his teaching was conducted at the Botanic Gardens, Glasnevin, where he had a laboratory until the administration of the gardens was reorganised in 1906. In 1891 he was requested to establish botanical collections (including a herbarium and specimens of economic significance) in the new Science and Art Museum in Dublin. He served as keeper of the botanical collections until 1923; thus, he was the founding director of the National Herbarium (rehoused in 1970 at the National Botanic Gardens, Glasnevin). In setting up the collection he solicited numerous materials internationally, thereby laying the basis for an important scientific and cultural heritage.
After the administrative transfer of both the college and the museum to the Department of Agriculture and Technical Instruction for Ireland in 1900, Johnson (effectively government botanist in Ireland) established and directed (until 1909) seed-testing and plant disease laboratories, the first official provision for seed testing in Britain and Ireland. In due course his researches led to the enactment of the Weeds and Agricultural Seeds (Ireland) Act, 1909. Johnson also inaugurated the scientific study of plant pathology in Ireland, subsequently sustained by several former students of the Royal College of Science. He was a member (elected 1893) and vice-president (1904–5) of the RIA. When the Royal College of Science (after some years of upheaval in the early 1920s) was transferred by an act of the oireachtas to UCD in 1926, Johnson remained as professor of botany (jointly with Joseph Doyle) until his retirement in 1928.
Johnson produced many scientific papers, chiefly on algae, plant pathology, and fossil plants. In the 1890s his publications dealt mainly with marine algae. During the early 1900s he made numerous pioneering investigations of plant diseases of economic significance. His travels in the west of Ireland on behalf of the Congested Districts Board stimulated his awareness of the value of botanical research applied to practical problems. His researches on plant pathology (such as gooseberry mildew, smut in oats, willow canker, wart disease of potato, and diseases of root crops) were extensively published in the Scientific Proceedings of the Royal Dublin Society and in the society's Economic Proceedings. As government botanist, he simultaneously wrote popular accounts of plant pathogens and suggested control measures. Additionally, in the early 1900s he produced numerous pamphlets for the public on diverse aspects of economic botany, such as flax cultivation, willow growing, and the uses of seaweeds.
He was unquestionably the foremost botanist actively engaged with economic aspects of plant science in Ireland in the early years of the 20th century. Appreciating the significance of plant pathology, he appointed Dr George Pethybridge as an assistant in 1900, and ceded this line of research to him in 1909; Pethybridge subsequently established a notable reputation as a plant pathologist. Thereafter, Johnson contributed to the Scientific Proceedings of the Royal Dublin Society (1911–22) on fossil plants, especially the beautifully preserved pteridosperms from Old Red Sandstone deposits at Kiltorcan quarry in Co. Kilkenny; he also wrote on the Tertiary fossils recovered from a lignite core at Washing Bay, Co. Tyrone. His engagement in successive disparate scientific themes over some forty years as professor had the eventual effect of diminishing the longer-term scientific impact he might otherwise have achieved by more focused research. His scientific legacy in Ireland may perhaps be characterised retrospectively as more institutional than academic, setting in train some enduring trends in Irish botany; his scientific researches themselves have been superseded or forgotten.
He married Bessie Stratton Rowe (d. 1919), with whom he had two sons and a daughter. Reginald Johnson MD (1888–1941) was a lieutenant colonel in the Royal Army Medical Corps and died on active service. Thomas William Gerald Johnson (d. 1976) was a captain in the same corps and won the military cross; after the war he practised medicine. Thirza Mary Patricia (1894–1955) was also a medical practitioner. Johnson's several successive residences in Dublin are listed in Mollan. He died 9 September 1954 at 64 Terenure Road East, Dublin, and was interred at Deansgrange cemetery, Co. Dublin. There is a portrait photograph of Johnson in the collection of the National Herbarium, National Botanic Gardens, Glasnevin, and in the Journal of the Department of Agriculture, xlviii (1951).