Johnson attended Elmfield College in Heworth, York and, after additional study, received an appointment in 1890 as Professor of Botany at Ireland's largest institution of higher learning, Catholic University of Ireland which, in 1909, became University College Dublin. In his 36-year tenure in the post he instructed generations of future botanists and other scientists, oversaw the expansion of the National Herbarium and was instrumental in the founding of the first seed testing station for Britain and Ireland. He also supervised and conducted research on economic botany, served on committees charged with advancing scientific learning in Ireland and was a pioneer in the study of phytopathology, the scientific study of plant diseases.
"JOHNSON, Thomas ... married a dau of Rev. W. Rowe, formerly of Toronto; one s (son died on active service) one d . ; Royal College of Science, London. Work: Demonstrator of Botany, 1885-90, Professor of Botany ..."--Who Was Who