© 2020 American Chemical Society. Magnesium borohydride ammonia borane, Mg(BH4)2(NH3BH3)2, was electrochemically investigated. Impedance measurements of the mechanochemically synthesized Mg(BH4)2(NH3BH3)2 exhibited an ionic conductivity of 1.3 × 10-5 S cm-1 at 30 °C. Electrochemical cells fabricated with Mg(BH4)2(NH3BH3)2 as the solid electrolyte demonstrated reversible Mg migration through the material, indicating its potential for use as a Mg ionic conductor in all-solid-state Mg-ion batteries.