Core double-shell cobalt/graphene//polystyrene nanocomposites (Co/C//PS) were synthesized by in situ sonochemical polymerization technique. Commercial Co/C nanoparticles are used and successfully lead to gram-scale production of processable nanocomposite. Synthesized Co/C//PS nanocomposites result in homogeneous and dense dispersion of particles with or without additional polymeric matrix. They showed improved thermal properties such as higher initial degradation temperatures and a significant increase of glass transition temperature (i.e. 10 to 12°C) in contrast to neat PS. These results suggest that covalent bonding occurs between PS and graphene shell, and may be promoted by two surface reactions: "grafting from" when monomer is pre-immobilized on graphene and grows to polymer, and "grafting to" when presynthesized polymer is immobilized on graphene. Both mechanisms are compared and explained. HR-TEM observations revealed polymer shells of 4 to 5 nm covering Co/C nanoparticles or at least small aggregates. However, the number of layer of the graphene shell which consists of 6 to 8 regular layers on raw particles decreases to 3, this layer reduction can be explained by a partial amorphization of graphene occurring during the polymerization. Nevertheless, Co particles are still efficiently protected from oxidation as final Co/C//PS nanocomposites are able to sustain high mass-magnetization (i.e. ~ 49 emu/g for 94 % wt Co/C). First indications of satisfying mechanical cohesion are also shown by the formation of two relevant nanocomposites shapes (film and disk). In conclusion, in situ polymerization is a powerful synthesis method to produce processable high-magnetization nanocomposites.