We report a solution-phase synthesis of 16 nm CuNi nanoparticles (NPs) with the Cu/Ni composition control. These NPs are assembled on graphene (G) and show Cu/Ni composition-dependent catalysis for methanolysis of ammonia borane (AB) and hydrogenation of aromatic nitro (nitrile) compounds to primary amines in methanol at room temperature. Among five different CuNi NPs studied, the G-Cu36Ni64 NPs are the best catalyst for both AB methanolysis (turnover frequency (TOF) of 49.1 molH2 molCuNi–1 min–1 and an activation energy (Ea) of 24.4 kJ/mol) and hydrogenation reactions (conversion yield of >97%). The G-CuNi compound represents a unique noble-metal-free catalyst for hydrogenation reactions in a green environment without using pure hydrogen.