The importance of mountain biodiversity protection and sustainable management is undisputed. With SDG indicator 15.4.1 on the coverage by protected areas of important sites for mountain biodiversity and with new targets agreed upon in the Kunming-Montreal Global Biodiversity Framework, needs and opportunities exist for meaningful reporting on mountain biodiversity protection. In providing SDG indicator 15.4.1 on mountain biodiversity protection for individual mountain ranges, we show that national-level indicators of mountain biodiversity protection provide little information at conservation-relevant scales and that reporting at subnational level is critical for the sustainable management of mountain ecosystems and for a meaningful interpretation of national values. Our work on SDG indicator 15.4.1 further highlights the need for transparency and caution with regard to the methods and assumptions underlying metrics for informing on ecosystem and biodiversity protection. Based on national and global datasets of mountain biodiversity monitoring efforts, which we map onto the ranges of the most recent GMBA mountain inventory, we also show that major gaps exist in the data available for reliable reporting on mountain biodiversity. We specifically show that even in mountain countries generally rich in environmental data, such as Switzerland, gaps in the monitoring of mountain ecosystems are numerous and large and undermine our ability to detect rapid changes in biodiversity. We argue that reliable reporting on global biodiversity-related targets at scale is not realistic unless we achieve improved spatiotemporal monitoring of mountain biodiversity and ecosystems. We further share examples of online resources to support science-based decision-making for complex mountain environments and the correct interpretation of the metrics on which decisions and management are based.