We investigated the electroless plating of magnetic metal nanoparticles on graphene to achieve nanoparticles with a tunable size and structure, a degree of exposure of the graphene surface, and magnetic properties of the hybrid material. FeCoNi ternary alloy nanoparticles were grown on the surface of graphene with various ratios of FeCoNi to graphene using two different electroless plating methods. The small size of the FeCoNi nanoparticles synthesized by conventional electroless plating resulted in a large amount of oxide and a low saturation magnetization of the hybrids. In contrast, the FeCoNi nanoparticles synthesized by dropwise electroless plating exhibited a large particle size and a high saturation magnetization. When the ratio of FeCoNi to graphene was increased, the saturation magnetization of both hybrids increased, but the size and structure of the nanoparticles were still different. These results imply that it is possible to synthesize tailored magnetic metal@graphene hybrid materials using an electroless plating method. This work provides guidelines for the preparation of metal nanoparticle@nanocarbon hybrid materials based on an electroless plating method.