This analysis focusses on the diplomatic roles of Joaquim Nery Delgado (1835‒1908), a geologist who served for about 50 years in the Portuguese Geological Survey, created in 1857 as part of the cartographic programme of the Ministry of Public Works, Trade, and Industry. In recent years, increasing attention has been paid by specialists in both science and technology studies and International Relations on the relationship amongst science, technology, and diplomacy, especially after the Second World War and the ensuing Cold War. However, historically, the relationship between science and diplomacy has deeper roots, stretching back to the early modern period. In this context, Delgado’s work as a Survey geologist operating within the state bureaucracy encompassed occasional scientific, political, and economic mediating roles, notably as the Portuguese government’s official representative at international events and cartographic projects, as well as in dealings of colleagues and businessmen with the Portuguese government. His fluid diplomatic functions accompanied the re-enforcement of the state apparatus and its international relations in a difficult period of Portuguese foreign policy.