Two large-scale glacier detachments occurred at the peaks of the 2013 and 2015 CE melt seasons, releasing a cumulative 24.4–31.3 × 106 m3 of ice and lithic material from Flat Creek glacier, St. Elias Mountains, Alaska. Both events produced highly mobile and destructive flows with runout distances of more than 11 km. Our results suggest that four main factors led to the initial detachment in 2013: abnormally high meltwater input, an easily erodible glacier bed, inefficient subglacial drainage due to a cold-ice tongue, and increased driving stresses stemming from an internal redistribution of ice after 2011. Under a drastically altered stress regime, the stability of the glacier remained sensitive to water inputs thereafter, culminating in a second detachment in 2015. The similarities with two large detachments in the Aru mountains of Tibet suggest that these detachments were caused by a common mechanism, driven by unusually high meltwater inputs. As meltwater production increases with rising temperatures, the possible increase in frequency of glacier detachments has direct implications for risk management in glaciated regions.