Groundwater depletion is a global concern. Around the world, groundwater supplies more than half the water used for agriculture and human drinking. Many other species and ecosystems are supported by groundwater and rely on the integrity of groundwater and surface water connections. Like many social and environmental problems, addressing the overextraction of groundwater requires collective action across governmental authorities and jurisdictions. To date, there are few examples of successful, voluntary groundwater management. To steer collective action at the local level, higher levels of government often use policy mandates. This dissertation examines the implementation of one such mandate. California’s Sustainable Groundwater Management Act (SGMA), a state-legislated mandate, was passed in 2014. SGMA requires local governmental agencies to work together to address decades of groundwater depletion. From 2018-2022, I conducted ethnographic fieldwork to study the mandated groundwater management planning processes that were undertaken by hundreds of local governmental agencies who, for the first time in California’s history, were faced with a choice: either they work across their jurisdictions to achieve groundwater sustainability or they forfeit their local control of groundwater resources to the state of California. Using a comparative case analysis approach, I address three core topics that are currently underexplored in research on mandated policy implementation: what motivates local governmental agencies to engage in collective action when under a mandate, how do local governmental agencies interact with one another to achieve mandate requirements, and why do we see variation in the ways local governmental agencies interpret their role in mandate implementation.