Mr. López de Silanes began his professional activity as a teacher at the Polytechnic University of Madrid's Escuela Técnica Superior de Ingenieros Industriales, and in 1971 he joined Compañía Arrendataria del Monopolio de Petróleos (CAMPSA), where he held various posts as Chief Project Engineer and Technical Area Manager.
In 1975 he worked in Madrid as an engineer for the state petroleum company Campsa, joining PSOE in the same year even though it was an illegal party until February 1977.