The unique two-dimensional structure and very high surface area of graphene results in amazing properties and makes it an ideal substrate for the chemical adsorption of many types of metal nanoparticles (NPs). Here, we demonstrate a novel approach for synthesizing multi-functional graphene-magnetic nanoparticles (GNPs) hybrids. The hybrids exhibit a combination of features, including excellent processability, superparamagnetism, electrical conductivity and high chemical reactivity. The synthesis of graphene by Cu or Ni reduction of exfoliated graphene oxide results in the removal of oxygen functionalities of the graphene oxide. The process is scalable, green and efficiently enables the controllable production of GNPs. The GNPs have great potential for a variety of applications, including as materials for magnetic resonance imaging, microwave absorption and electromagnetic interference shielding.