Oxidized multi-wall carbon nanotubes (MWCNTs) are decorated with iron oxide (Fe3O4) nanoparticles using an ex situ and simpler single-step in situ approach (synthesis of ex situ and in situ MWCNT-Fe3O4). The polar carboxylic acid moieties present on the MWCNT backbone, as a result of oxidation, acted as anchoring sites through which the nanoparticles could be attached. The remaining carboxylic acid moieties present on the backbone facilitated dissolution in polar solvents (such as dimethylformamide (DMF)); which allowed for incorporation of the respective MWCNT-Fe3O4 samples as filler nanomaterials into nanofibres using the electrospinning technique. Excellent dispersion and alignment of the respective MWCNT-Fe3O4 samples along the poly(methyl methacrylate) (PMMA) electrospun nanofibre axis were obtained. Highest values for magnetization were exhibited by the electrospun composite nanofibres containing ex situ synthesized MWCNT-Fe3O4; where these composites are proposed for use in SMART polymer applications such as tissue scaffolds or magnetic filters.