LiBH₄–MgH₂ system in a 2 : 1 molar ratio constitutes a representative reactive hydride composite (RHC) for hydrogen storage. However, sluggish kinetics and poor reversibility hinder the practical applications. To ease these problems, amorphous TiB₂ and NbB₂ nanoparticles were synthesized and employed as catalysts for the 2LiBH₄–MgH₂ system. Isothermal de-/rehydrogenation and temperature programmed mass spectrometry (MS) measurements show that amorphous TiB₂ and NbB₂ nanoparticles can significantly improve the hydrogen storage performance of the 2LiBH₄–MgH₂ system. 9 wt% hydrogen can be released within only 6 min for nanoTiB₂-doped 2LiBH₄–MgH₂, while for the undoped composite limited hydrogen of 3.9 wt% is released in 300 min at 400 °C. The dehydrogenation activation energies for the first and second steps are dramatically reduced by 40.4 kJ mol⁻¹ and 35.2 kJ mol⁻¹ after doping with nanoTiB₂. It is believed that TiB₂ and NbB₂ nanoparticles can first catalyze the dehydrogenation of MgH₂, and then induce the decomposition of LiBH₄ and meanwhile act as nucleation agents for MgB₂, thereby greatly enhancing the kinetics of dehydrogenation. The present study gives clear evidence for the significant performance of transition metal boride species in doped RHCs, which is critically important for understanding the mechanism and further improving the hydrogen storage properties of RHCs.