In Thailand, mangrove forests are claimed for state management, although it is widely recognized that coastal communities access and manage those forests. Skepticism persists within Thai government circles about whether coastal villages can sustainably manage and protect mangroves. This research presents evidence of successful mangrove conservation and management by two coastal villages in Trang province, southern Thailand. Using interdisciplinary methods including interviews, discussions, quantitative forest surveys, and institutional analysis, we describe the history of how these two communities gained rights to manage the mangrove forests, and the subsequent positive biological outcomes associated with their management. Local villages have crafted and maintained well-defined governance management institutions over the forest, and as a result, stand structure was superior in community-managed mangrove forests than in the open-access state forest. We argue that the basis for the communities’ success in managing these forests was that the resource was necessary to local livelihoods and was becoming scarce; the communities enjoyed autonomous decision making and had a high degree of social capital; the forest and user groups were well defined and monitored; effective leadership was present in the villages to apply sanctions and resolve conflicts; and there was substantial assistance from an external non-governmental organization, which served as a bridge between the villages and the government. For conservation, simply knowing whether communities can conserve mangroves is not sufficient. We must know why the communities are or are not successful. Conservation research must, therefore, consider not only the biological outcomes of community management, but also the underlying reasons for those outcomes. This paper can serve as a guideline for future studies on the community-mangrove interface (coastal development).