Mg(BH4)2 exhibits a high hydrogen content of 14.9 wt% and thermodynamic stability in the overall decomposition reaction that corresponds to hydrogen desorption at around room temperature. However, the potential applications in hydrogen storage are restricted by high kinetic barriers. In this study, we show the synthesis of Mg(BH4)2/carbon nanocomposites by ball milling of MgH2 nanoparticles supported on carbon aerogel in a B2H6/H2 atmosphere. The nanocomposite exhibits a lower kinetic barrier as compared to bulk Mg(BH4)2. The temperature for major hydrogen desorption is decreased to 160 °C. Furthermore, re-formation of Mg(BH4)2 after full dehydrogenation is achieved under mild conditions (200 °C and 80 to 150 bar H2) in the nanocomposite. This work demonstrates nanoengineering as an effective approach to realize the reversible hydrogen storage of Mg(BH4)2 under mild conditions.