Glacial lake outburst floods (GLOFs) are common where highly dynamic temperate glaciers exist, since seasonal changes in ice-conduit dynamics can start rapid lake drainages. Lakes dammed by cold-based glaciers, however, are less common and GLOFs from these glaciers have been rarely reported. Understanding both the origin and the failure mechanisms of lakes dammed by cold-based glaciers and subsequent flood processes is essential for territorial planning. We study a remarkable GLOF triggered by the failure of a subglacial lake in the Manflas Valley, Arid Andes of Chile, in 1985 providing insights into the lake's origin, clarifying the failure mechanism and modelling the GLOF event-related dynamics. To identify the factors that contributed to the lake formation and failure, we analysed remotely sensed images, meteorological and topographic data. The GLOF dynamics were reconstructed using empirical (LAHARZ and MSF) and physical models (RAMMS). The obtained results were compared with field data of flow extent, depth and velocity. We show that the failed lake (4 × 106 m3) formed in a low-slope (≤ 10°) area and that extreme (≥ 90th percentile) annual precipitation before the GLOF contributed to the lake filling and probably to the dam collapse. The lake likely drained rapidly after mechanical failure of the ice-dam producing a high energy sediment-laden flow. We show the challenges of modelling large flows over long distances (dozens of kilometres) especially when flows change between Newtonian and Non-Newtonian phases. A GLOF can still endanger the Manflas Valley since a remnant of the lake of about 220.000 m3 exists and economic assets are located along the1985 GLOF path.