The morphology of the Mangala Fossa graben forming the source of the Mangala Valles implies that two episodes of graben subsidence took place, each induced by lateral dike intrusion from Arsia Mons. Quantitative modeling suggests that graben boundary faults breaching the cryosphere provided pathways for water release from an underlying aquifer at a peak rate of ∼107 m3 s−1. In the first event, the graben subsided by ∼200 m, and water carrying a thin ice layer filled the graben, overflowing after ∼2.5 hours, mainly at a low point on the north rim. This captured the water flux, eroding a gap in the north wall which, with an erosion rate of ∼100 μm s−1 and a duration of ∼1 month, was ∼250 m deep by the end of water release. Erosion of the graben floor also took place, at ∼20 μm s−1, lowering it by ∼50 m. Subsequently, heat from the cooling dike melted cryosphere ice, causing a further ∼150 m of subsidence on compaction. In the second event, with a similar duration and peak discharge, the graben again subsided by ∼200 m and filled with ice‐covered water until overflow through the gap began at a water depth of ∼350 m. The gap was eroded down by a further ∼400 m, and the floor was eroded by a further ∼50 m. Finally, heat from the second dike sublimed cryosphere ice, lowering the floor by ∼100 m. In places, combined erosion and subsidence of the graben floor exposed ∼200 m of the first dike.