For more than a decade, non-governmental organisations (NGOs) have been studied by the social and political sciences. From a geographic point of view, however, it is surprising that the role of the socio-spatial contextuality of NGO action has not yet been explored in a systematic way. Taking the example of transnational NGOs in Brussels and Geneva, this article contributes to bridging this research gap. By cross-referencing to the global city debate, it is argued that civil society functions are as relevant for the formation of global cities as economic functions. It is also suggested that the role of the socio-spatial context conditions, under which the NGO actors operate, has to be considered in the analysis of NGO influence. On the basis of structuration theories this institutionally dense context is conceptualized as a socio-spatial opportunity structure. Finally, this argumentation is supported by empirical evidence from case studies in the two cities.