This study aimed to build on theory and practice regarding the combinations of conditions that influence water service sustainability when external partners are involved. The study investigates 26 well projects that have been implemented in developing countries with the assistance of Engineers Without Borders-USA (EWB-USA). Using past literature on sustainable water service delivery in developing communities, emergent coding techniques with project documents, and surveys with EWB-USA team members, this study identifies a set of project conditions to conduct fuzzy-set Qualitative Comparative Analysis (fsQCA). Findings show that the presence of a water committee cannot alone account for project sustainability. Additional conditions, such as technology and construction processes, project governance, and community engagement practices must also be considered for project sustainability. The relationship between construction quality and financial sustainability is also discussed. Overall, the findings from this research contribute to sector theory and reveal distinct pathways towards sustainable water services. These findings informed recommendations for EWB-USA well project implementation and management, and demonstrate the utility of fsQCA as a tool to navigate the complexities of water service delivery by external partners and improve understanding to increase water service sustainability.