In this paper we report results on both material preparation and characterization of a polystyrene-based magnetic nanocomposite material. CoFe2O4 mineralized sulfonated polystyrene was prepared by in-situ precipitation and oxidation of Co+2 and Fe+2 within a sulfonated polystyrene resin. The magnetic oxide particles of nanometer size were characterized by wide angle X-ray diffraction (WAXD). Polystyrene-based composite films were obtained by dispersion of the finely milled mineralized resin in a polystyrene matrix from the melt state (compositions ranging from 0 to 50 wt% of mineralized resin). The thermal and mechanical properties of the nanocomposite films were determined by TGA, DSC, and stress-strain testing. The magnetic characterization of the samples was also performed. No significant changes in thermal stability, glass transition temperature or mechanical properties of the polystyrene matrix occur as a consequence of the content in mineralized resin. These results show that the filler acts as an inert diluent for the polymer matrix. On the other hand, the magnetic characterization of the samples reveals the presence of nanoparticles with diameters ranging from 3 to 8 nm, showing, at room temperature, coercive field values of around 800 Oe and saturation magnetization between 120 and 420 emu/g. The combination of these properties suggests the use of these systems in the preparation of magnetic recording materials with high recording density.