Atmospheric CO2 is naturally sequestered in ultramafic mine tailings as a result of the weathering of serpentine minerals [Mg3Si2O5(OH)4] and brucite [Mg(OH)2], and subsequent mineralization of CO2 in hydrated magnesium carbonate minerals, such as hydromagnesite [Mg5(CO3)4(OH)2·4H2O]. Understanding the CO2 trapping mechanisms is key to evaluating the capacity of such tailings for carbon sequestration. Natural CO2 sequestration in subaerially exposed ultramafic tailings at a mine site near Mount Keith, Australia is assessed with a process-based reactive transport model. The model formulation includes unsaturated flow, equations accounting for energy balance and vapor diffusion, fully coupled with solute transport, gas diffusion, and geochemical reactions. Atmospheric boundary conditions accounting for the effect of climate variations are also included. Kinetic dissolution of serpentine, dissolution-precipitation of brucite and primary carbonates—calcite (CaCO3), dolomite [MgCa(CO3)2], magnesite (MgCO3), as well as the formation of hydromagnesite, halite (NaCl), gypsum (CaSO4·2H2O), blödite [Na2Mg(SO4)2·4H2O], and epsomite [MgSO4·7H2O]—are considered. Simulation results are consistent with field observations and mineralogical data from tailings that weathered for 10 yr. Precipitation of hydromagnesite is both predicted and observed, and is mainly controlled by the dissolution of serpentine (the source of Mg) and equilibrium with CO2 ingressing from the atmosphere. The predicted rate for CO2 entrapment in these tailings ranges between 0.6 and 1 kg m−2 yr−1. However, modeling results suggest that this rate is sensitive to CO2 ingress through the mineral waste and may be enhanced by several mechanisms, including atmospheric pumping.