© 2020 Hydrogen Energy Publications LLC A phase stability map of metallic magnesium powder, exposed to environmental conditions for 12 months (Mg-12M) and subjected to different high-energy ball-milling speeds and milling times, was constructed. Mg-12M−160 [½MgO-⅓Mg(OH)2-⅙hydromagnesite] and Mg-12M−640 [¼MgO-⅝Mg(OH)2-⅛hydromagnesite] composites were obtained changing the milling conditions. The correlation among the accumulated energy (ΔEaccum), the impact energy (ΔEhit), and the phase stability under different high-energy ball-milling conditions were generated. The Mg-12M−160 composite had a hydrogen storage capacity of 0.63 wt% at −196 °C and 8.3 bar, although further hydrogen adsorption at higher pressures is expected. Structural defects play a significant role in the adsorption capacity. A representation of the possible absorption mechanism is proposed.