A Mg(BH₄)₂-added Mg(NH₂)₂–2LiH system was prepared by ball milling the corresponding chemicals. The hydrogen storage properties of the Mg(NH₂)₂–2LiH–xMg(BH₄)₂ (x = 0, 0.1, 0.2, 0.3) samples and the role played by Mg(BH₄)₂ were systematically investigated. The results show that the onset and peak temperatures for hydrogen desorption from the Mg(BH₄)₂-added Mg(NH₂)₂–2LiH sample shifted to lower temperatures. In particular, the Mg(NH₂)₂–2LiH–0.1Mg(BH₄)₂ sample could reversibly absorb 4.5 wt% of hydrogen in the temperature range of 120–150 °C, which is superior to the pristine sample. During ball milling, a metathesis reaction between Mg(BH₄)₂ and LiH readily occurred to form LiBH₄ and MgH₂ and subsequently, the newly formed MgH₂ reacted with Mg(NH₂)₂ to generate MgNH. Upon heating, the presence of LiBH₄ not only decreased the recrystallization temperature of Mg(NH₂)₂ but also reacted with LiNH₂ to form the Li₄(BH₄)(NH₂)₃ intermediate, which weakens the N–H bonding and enhances the ion conductivity. Meanwhile, MgNH may act as the nucleation center for the dehydrogenation product of Li₂MgN₂H₂ due to the structural similarity. Thus, the in situ formed LiBH₄ and MgNH provide a synergetic effect to improve the hydrogen storage performances of the Mg(NH₂)₂–2LiH system.