Sustainability of water supply has over the years become a key concern for the global community due to water stress and scarcity attributable to climate change and other anthropogenic factors. This is especially crucial for urban areas where increased rural-urban migration has brought population increases, thus raising the demand for basic services and infrastructure. The concerted efforts of water sector stakeholders have seen to, among other strategies, the implementation of water supply projects in affected areas. In Kenya, however, the statistics show that these water supply projects have reported high failure rates with the existing systems failing to operate at full capacity. The study sought to investigate the effect on the effect of accountability on the sustainability of water supply projects in Nairobi City County's informal settlement areas. A sample of 260 project leaders of water supply projects was considered in this study. Data was collected using structured questionnaires, and both descriptive and inferential data analysis was conducted. The findings revealed that accountability has a positive and statistically significant effect on sustainability and that strengthening accountability practices such as monitoring and evaluation, managerial evaluation, and transparency would bolster project sustainability.