The well-being and mental health of principals is being threatened by changing working conditions such as a broader variety of roles and tasks. In this article, we argue that social capital might buffer against declining (mental) health. The purpose of this study was to investigate the potential influence of social capital, including both internal (bonding) and external (bridging and linking) social capital, on principals' well-being. A longitudinal study was set up across three waves of data. Questionnaires were collected from 2084 Australian principals and 829 Irish principals, across six and two timepoints, respectively. The hypotheses were tested using a longitudinal path model approach using maximum likelihood estimation in lavaan for R. In line with our hypotheses, the results indicated that principals who reported higher levels of either internal or external social capital also reported higher levels of well-being. More specifically, support from colleagues outside the school and supervisor support (external social capital) and collaboration and trust in management (internal social capital) predicted well-being positively across time. The results of this study highlight the importance of having social support from colleagues and supervisors and possibilities for collaboration to maintain well-being as a school principal.