In the paper, we synthesized amorphous NiCoB nanoparticles by a simple chemical reduction method and employed them as high-activity catalysts to considerably improve the hydrogen storage properties of MgH2. The MgH2-NiCoB composite quickly absorbed 3.6 wt % H2 at a low temperature of 85 °C and released 5.5 wt % H2 below 270 °C within 600 s. It is worth noting that the hydrogenation activation energy was reduced to 33.0 kJ·mol-1. Detailed microstructure analysis reveals that MgB2, Mg2Ni/Mg2NiH4, and Mg2Co/Mg2CoH5 were in situ generated during the first de/absorption cycle and dispersed at the surface of NiCoB. These active ingredients created lots of boundary interfaces to facilitate the hydrogen diffusion and destabilize the Mg-H bonds, thus decreasing the kinetic barriers. This work provides support for a promising catalytic effect of amorphous NiCoB on de/absorption reactions of MgH2, showing new ways for designing Mg-based hydrogen storage systems toward practical application.