The decomposition properties of Mg(BH4)2−LiNH2 mixtures were investigated. Apparent NH3 release appeared from 50 to 300 °C for the Mg(BH4)2−LiNH2 mixtures with mole ratios of 1:1.5, 1:2, and 1:3, while only hydrogen release was detected for the mixture with a mole ratio of 1:1. In the case of the Mg(BH4)2−LiNH2 (1:1) sample, the onset of the first-step dehydrogenation starts at 160 °C, with a weight loss of 7.2 wt % at 300 °C, which is improved significantly compared to the pure Mg(BH4)2 alone. From Kissinger’s method, the activation energy, Ea, for the first and second step dehydrogenation in Mg(BH4)2−LiNH2 (1:1) was estimated to be about 121.7 and 236.6 kJ mol−1, respectively. The improved dehydrogenation in the combined system may be ascribed to a combination reaction between [BH4] and [NH2], resulting in the formation of Li−Mg alloy and amorphous B−N compound.