||Magnesium borohydride (Mg(BH4)2, abbreviated here MBH) has recently emerged as a potentially promising material for carbon dioxide capture and hydrogen storage applications due to its naturally porous structure, high theoretical internal surface area, high gravimetric hydrogen content, and high CO2 uptake. However, recent reports for MBH have shown synthetic limitations in terms of yielding desired phase-pure MBH. Consequently, meeting their theoretical physical properties has proven challenging. Here, I demonstrate effective synthetic strategies to obtain pure polymorphic phases of MBH nanomaterials supported by reduced graphene oxide (MBHg) under mild conditions (60 - 190 °C under low vacuum, 2 Torr). Moreover, MBHg nanomaterials exhibit increased CO2 uptake and larger amount of H2 release compared to the parent bulk materials, which was investigated by chemical pathways, microstructural features, as well as structural stability and oxidation resistivity.