Scholarship has traditionally portrayed transnational NGOs (TNGOs) as ‘principled’ actors animated by global norms to advance human rights, sustainable development, humanitarian relief, environmental stewardship, and conflict resolution. However, scholarship has also identified instances in which TNGOs appear to act ‘instrumentally’ by engaging in resource-maximising behaviour seemingly inconsistent with their principled nature. Moreover, prior scholarship addressing this puzzle has been constrained by the limitations of small-n case studies examining relatively narrow subsectors of the TNGO community. Addressing these limitations, we reexamine the logic of TNGO behaviour in light of findings from an interdisciplinary, mixed-method research initiative consisting of in-depth, face-to-face interviews with a diverse sample of 152 top organisational leaders from all major sectors of TNGO activity. Using an inductive approach to discover how TNGO leaders understand their own behaviour, we introduce the heuristic of ‘principled instrumentalism’ and specify our framework with a formal model.