Vegetation fire generates vast amounts of mineral ash annually that can be readily mobilized by water or wind erosion. Little is known about the photoactivity of dissolved mineral ash in aquatic systems and its ability to mediate redox reactions of environmental pollutants. This study reports that dissolved mineral ash derived from pyrolysis of biomass is photoactive under simulated sunlight, generating reactive oxygen species. It can mediate the photoreduction of hexavalent chromium (Cr(VI)) in the presence of electron donors; for example, phenols and dissolved organic matter, at pH 4.7. The reaction kinetics followed the Langmuir-Hinshelwood model, suggesting a heterogeneous photocatalytic reaction. The enhancement of reduction efficiency was linearly correlated with the one-electron reduction potential of phenols. The synergy between dissolved mineral ash and phenols is attributed to the inhibition of electron-hole recombination. The reduction rate decreases with increasing solution pH, owing to the decreased reduction potential and surface adsorption of Cr(VI). The silicon and silicon carbide components are most likely responsible for the photocatalytic activity of dissolved mineral ash. Our results suggest that dissolved mineral ash is a natural photocatalyst that can mediate redox reactions of pollutants in sunlit aquatic systems, playing an overlooked role in natural attenuation and aquatic photochemistry.