Environmental decentralization has been promoted over the past 40 years to improve policy efficiency and fairness. However, the outcomes have been mixed. While some countries have found positive results, decentralization has led to overexploitation and increased pollution spillovers due to administrative and political fragmentation, particularly in the Global South. Scholars usually attribute the drawbacks of environmental decentralization to corruption and rent-seeking, the blame-avoidance opportunities created by blurred administrative borders, and political misalignment across government levels. One overlooked dimension is the role of bureaucratic politics in explaining the outcomes of decentralization. This paper explores how the structure and politics of the bureaucracy determine the effectiveness of environmental decentralization. I analyze public organizations’ professionalization and politicization as drivers of the state’s capacity to implement and coordinate environmental decentralization. Furthermore, I argue that bureaucratic professionalization reduces risks of local political capture, enabling decentralization’s benefits. However, excessive professionalization may hinder flexibility and innovation. Using dynamic panel methods on a sample of ~90 countries from 2002 to 2020, I show that, conditional on the politicization and professionalization of the bureaucracy, environmental decentralization has heterogeneous and non-linear effects on environmental outcomes. This research sheds light on the public sector reforms required to capture the potential advantages of environmental decentralization.