The advantage of potassium–magnesium bimetallic hydride as novel destabilization agent for LiNH2is demonstrated by a comparative study among MgH2-, NaMgH3-, and KMgH3- systems. LiNH2combined with KMgH3not only results in a substantial decreased desorption temperature by >60 °C and suppression of NH3release but also enhances sorption kinetics two to five times in comparison with the single MgH2or NaMgH3. A high reversible capacity of >65% is also achieved, even recharging under milder conditions. Through detailed structural analysis on the desorption products in various stages of these combined systems, we proposed a “self-concerted” reaction mechanism in which the in situ formed active MgH2and KH by KMgH3are involved in LiNH2decomposition. In addition, the remarkable promotion in conveyance of N-containing species by highly active KH compared with NaH is believed to be responsible for the superior destabilizing effect of KMgH3over NaMgH3in improving hydrogen sorption kinetics of LiNH2.