The effect of Al addition on the reversibility of a MgH₂+2LiBH₄ hydrogen storage mixture was examined in order to improve the mixture’s requirement for a hydrogen atmosphere even in dehydrogenation. The experiments using high pressure differential scanning calorimetry and X-ray powder diffraction confirmed that a MgH₂+Al+4LiBH₄ mixture can reversibly dehydrogenate and rehydrogenate below 773 K under mild conditions of 0.1 MPa H₂ for dehydrogenation and 4.0 MPa H₂ for rehydrogenation. Moreover, thermogravimetry tests revealed that this mixture starts hydrogen desorption at about 530 K, which is 80 K lower than the corresponding temperature for the MgH₂+2LiBH₄ mixture, and desorbs 9.5 mass% of hydrogen below 773 K. Thus, the addition of Al improves not only the reversibility of the reaction but also dehydrogenation kinetics. The hydrogen desorption of the mixture occurs by three steps, which includes the formation of Mg-Al alloys by the reaction of MgH₂ and metallic Al followed by the formation of Mg_1−xAl_xB₂ by the reaction of the Mg-Al alloys and LiBH₄. Al in this mixture suppresses the formation of metallic Mg and accelerates the formation of Mg_1−xAl_xB₂ from B produced by dehydrogenation of LiBH₄. Mg_1−xAl_xB₂ is derived from partial substitution of Al for Mg in MgB₂, which contributes to reversible hydrogenation and dehydrogenation of MgH₂+2LiBH₄.