This work aimed to reduce the steps used for preparing homogeneous and functionalized magnetic polymeric nanoparticles (MPNP) under mild conditions. A Fe3O4 magnetic nanoparticle (MNP) was synthesized by the chemical co-precipitation of an aqueous Fe3+/Fe2+ solution with NH4OH at room temperature under N2 atmosphere. After dispersing an MNP coated with oleic acid into a mixture of styrene (St) and acrylic acid (AA) monomers, the MPNP was functionalized with AA and was produced through the miniemulsion polymerization in the absence of a magneto-template. When mixing a crosslinking agent (divinyl benzene; DVB) with St and AA in the polymerization process, we obtained a homogeneous distribution of MNP (53%) in the PS/DVB matrix without the appearance of a non-magnetic polymer particle. The superparamagnetic MPNP, therefore, fulfilled the criteria for further use in bio-related applications.