The integration of carbon capture and storage (CCS) technologies with energy conversion processes will increase the sustainability of using carboneous fuels. Combined CCS via carbon mineralization, where CO₂ reacts to form solid, stable mineral carbonates represents an attractive solution that avoids the energy costs of sorbent regeneration and public and scientific inquiries surrounding the safety of long-term geological storage of CO₂ in subsurface reservoirs. This study investigated the integration of slurry phase Mg(OH)₂ carbonation with the water gas shift reaction (WGSR) at temperatures up to 215 °C with a 0.5 wt% platinum on alumina catalyst. The removal of CO₂ by the carbonation reaction enhanced hydrogen yield of the WGSR as the equilibrium of the gas phase reaction was shifted towards products. Aqueous Mg(OH)₂ carbonation has been shown to occur readily, and various metastable and stable magnesium carbonates are formed depending on the reaction conditions (primarily driven by temperature). This study revealed that the H₂ yield was enhanced in the presence of Mg(OH)₂ slurry, and more interestingly the unexpected conversion of CO to aqueous formate ion was observed, which did limit the overall production of H₂. The presence of the basic aqueous phase within the reactor promoted a side reaction between CO and hydroxide ions. The formate can persist in solution or decompose to yield H₂ depending on the solution conditions. The solid and liquid components of the reacting slurry were carefully analyzed for carbonate and formate in order to investigate the reaction mechanisms.