High operating temperature and slow kinetics retard the practical applications of the Li−Mg−N−H system for hydrogen storage. To alleviate these problems, a first attempt was carried out by synthesizing Li₂MgN₂H₂ through sintering a mixture of Mg(NH₂)₂−2LiNH₂ and investigating its size-dependent hydrogen storage performance. A dramatically enhanced kinetics for hydrogen absorption/desorption was achieved with a reduction in the particle size. For the dehydrogenation reaction, a three-dimensional diffusion-controlled kinetic mechanism was identified for the first time by analyzing isothermal hydrogen desorption curves with a linear plot method. The experimental improvement and mechanistic understanding on the dehydrogenation kinetics of the Li−Mg−N−H system shed light on how to further decrease the operating temperature and enhance the hydrogen absorption/desorption rate of the amide/hydride combined materials.