The cycling of hydrogen plays an important role in the geochemical evolution of our planet. Under high-pressure conditions, asymmetric hydroxyl bonds tend to form a symmetric O-H-O configuration in which H is positioned at the center of two O atoms. The symmetrization of O-H bonds improves their thermal stability and as such, water-bearing minerals can be present deeper in the Earth's lower mantle. However, how exactly H is recycled from the deep mantle remains unclear. Here, we employ first-principles free-energy landscape sampling methods together with high pressure-high temperature experiments to reveal the dehydrogenation mechanism of a water-bearing mineral, FeO2H, at deep mantle conditions. Experimentally, we show that ∼50% H is released from symmetrically hydrogen-bonded ε-FeO2H upon transforming to a pyrite-type phase (Py-phase). By resolving the lowest-energy transition pathway from ε-FeO2H to the Py-phase, we demonstrate that half of the O-H bonds in the mineral rupture during the structural transition, leading toward the breakdown of symmetrized hydrogen bonds and eventual dehydrogenation. Our study sheds new light on the stability of symmetric hydrogen bonds during structural transitions and provides a dehydrogenation mechanism for hydrous minerals existing in the deep mantle.