Flash floods in mountainous regions are becoming more frequent and destructive under climate warming, yet cross-regional understanding of their triggering mechanisms, cascading impacts, and governance remains fragmented. This review synthesises 1,967 studies published during 2000–2025 to establish a globally comparable baseline of mountain flash-flood research. By integrating bibliometric and topic analyses with qualitative synthesis, we reveal pronounced geographical and thematic imbalances, with research concentrated in Europe and Asia. At the same time, many high-risk mountain regions in Africa and South America remain overlooked. Across regions, flash-flood initiation and impacts are shown to be strongly state-dependent and coupled, emerging from interactions between storm intensity, duration and spatial concentration, antecedent hydrological conditions, and hillslope-channel connectivity. This coupling helps explain why fixed rainfall thresholds are difficult to generalise and highlights the need for dynamic, multi-source early-warning approaches. Comparing evidence on early warning, structural protection, and Nature-Based Solutions, the review shows that cascading processes dominate risk management challenges. We therefore propose an adaptive governance framework that links monitoring and forecasting, spatial planning, grey-green integration, and basin-scale risk sharing under non-stationary climate conditions. Overall, this synthesis consolidates fragmented evidence into a cross-regional knowledge base to support flash-flood risk reduction in mountainous regions, where data and capacity are limited.