Tumi Tómasson received his Fil. Kand. in ecological zoology from the University of Umeå, Sweden in 1976 and a M.Sc. in fisheries science from Oregon State University, USA, in 1978. In 1978-1979 he worked as a specialist for the Institute of Freshwater Fisheries before joining the J.L.B. Smith Institute of Ichthyology in Grahamstown, South Africa, as a research officer in mid 1979. He did research on the adaptation of large riverine cyprinids to man-made lakes and completed his Ph.D. from Rhodes University in South Africa in 1983.
He headed the northern division of the Institute of Freshwater Fisheries in 1983-1998, with a five-year interruption while he worked for the Icelandic International Development Agency in Malawi, first as an adviser to the SADC Regional Fisheries Training Programme, and later as a research consultant for stock evaluation on Lake Malawi.
He moved back to northern Iceland in 1996 and worked there for two years before becoming the director of the UNU Fisheries Training Programme (UNU-FTP) when it was established in 1998. In 2008-2009 he took leave for 15 months while working as a country director for the Icelandic International Development Agency in Sri Lanka. Tumi has served on a number of committees and boards and has worked as a consultant in Iceland and internationally.