Tomás started his career as an activist lawyer against the dictatorship of General Franco in Spain (1969-1979). He then spent nine years working as a lecturer in Sociology at the University of Wisconsin (Madison, US) and as a sociologist in the regional governments of Andalusia and Madrid in Spain.
In 1989 he joined the European Commission in Brussels, where he was successively desk officer for Palestine and Lebanon (1989-1992), head of Sector “Occupied Territories of the West Bank and Gaza Strip” (1992-1993), European Commission Representative to the Occupied Territories and Head of the European Commission Technical Assistance Offices in Jerusalem and Gaza (1994-1996), head of the Mashreq and Israel Unit (1996-2001) and head of the Latin America Horizontal Affairs Unit (2001-2004).
From January 2004 to September 2007 he was Director for Latin America in the Directorate General for External Relations of the European Commission. Between January and September 2007, he was Acting Deputy Director-General for Asia and Latin America. From October 2007 to January 2011, he was Director, South Mediterranean and Middle East. In February 2011, he became Director for the Americas in the newly established European External Action Service. He retired in February 2013.