Globally, hundreds of thousands of rangers patrol protected areas every day. The data they collect have immense potential for monitoring biodiversity and threats to it. Technologies like SMART (Spatial Monitoring and Reporting Tool), which facilitate the management of ranger-collected data, have enhanced this potential. However, based on our experience across diverse use cases and geographies, we have found that ranger-based monitoring is often implemented without a clear plan for how the data will inform management and without critical evaluation of whether the data are reliable enough to meet specific monitoring goals. Here we distill six key lessons and present a decision framework to guide funders, governments, protected area managers, and NGOs toward more effective use of ranger-based monitoring for protected area management and suggest when alternative monitoring approaches may add value. Essential considerations include the welfare and motivation of rangers, biases in patrol coverage and detectability, the capacity to analyze data, and the buy-in of those tasked with using the data to inform management decisions. When implemented well, ranger-based monitoring can help improve conservation outcomes through evaluating management interventions, more efficient deployment of limited law enforcement budgets to optimize the deterrence of illegal activities, and basic ecological monitoring.