He initiated his professional career in 1971 in the first British Government research team in Malawi, where he developed new forage-based systems for small-scale beef production.
While working in South America, Derrick Thomas published extensively on forage research, including several articles together with his CIAT colleague Bert Grof (see below).
In 1989, he was appointed Head of the Plant Sciences Division and Co-ordinator of the Feed Resources and Resource Use Programmes at the International Livestock Centre for Africa (ILCA, now the International Livestock Research Institute, ILRI) in Addis Ababa, Ethiopia, another CGIAR center.
At ILCA he headed multi-disciplinary research teams in sustainable agriculture and natural resources management operating throughout sub-Saharan Africa.
At the NRI, he undertook short-term consultancy work in Asia, Latin America and the Caribbean, the Middle East and sub-Saharan Africa.