This article reintroduces the League of Nations Secretariat as a fundamentally significant object of historical study. By drawing on key insights from three generations of historiography on the Secretariat, the authors explore how historians can use a Bourdieusian conceptual framework to study this first major international administrative body. Each generation of literature has emphasized one of three professional archetypes – the bureaucrat, the diplomat and the technocrat. Moving beyond these archetypes, and applying Antoine Vauchez's concept of ‘weak fields’ and the notions of import, brokering capacity and hybridity, we see how the professional templates that were being imported into the Secretariat were culturally specific (mainly to Britain and Northern Europe) and how they were merged and reinvented to secure the smooth running of a multilateral, multinational and multivalent organization given charge of a series of new functions, thus producing new, specific forms of expertise exclusive to the Secretariat. Accordingly, we capture both the complexities of what kind of professional cultures came to dominate the Secretariat and the novelty of some of the types of expertise it rested upon: an important step towards a deeper understanding of the characteristics and role of international public administration in international politics in the twentieth century.