Well dispersed magnetically recyclable trimetallic core–shell Ag@CoNi nanoparticles (NPs) supported on graphene have been synthesized via a facile one-step in situ procedure using methylamine borane (MeAB) as the reducing agent. The as-synthesized NPs exhibit much higher catalytic activities for hydrolytic dehydrogenation of ammonia borane (AB) than the monometallic, bimetallic, trimetallic alloy (AgCoNi/graphene), and graphene free (Ag@CoNi) counterparts. Moreover, compared with NaBH₄ and AB, the weaker reducing agent MeAB has much better control during the synthesis of the graphene supported Ag@CoNi NPs, which resulted in the highest catalytic activity. Kinetic studies indicate that the catalytic hydrolysis of AB by the Ag@CoNi/graphene NPs is first order, with the activation energy measured to be 36.15 kJ mol⁻¹. Furthermore, the as-prepared NPs exert good catalytic activities and recycle stabilities towards the hydrolysis of AB and MeAB. Hence, this general method indicates that MeAB can be used as both a potential hydrogen storage material and an efficient reducing agent, and can be easily extended to facile preparation of other graphene supported multi-metal NPs.