Polymer-functionalized magnetic multiwalled carbon nanotubes (MWCNTs) have been prepared by a 2-step process. Fe3O4 nanoparticles are first decorated onto MWCNTs through Diels-Alder reaction. The resulting magnetic MWCNTs have been functionalized with polymers through an ozone-mediated process, which is able to incorporate kinds of matrix polymers to the magnetic MWCNTs. Poly(lactide)-functionalized magnetic MWCNTs (mCNT-PLA) have been prepared in this work for preparation of electrically-conductive and magnetic poly(lactide)/MWCNTs composite films and electrospun fibers. The magnetic and electrically-conductive feature of the functionalized MWCNTs helps to the formation of nanofibers in the electrospinning process of poly(lactide). The nanofibers possessing 0.3 wt% of mCNT-PLA exhibit a Young's modulus and an elongation at break of 25 MPa and 150%, respectively. Compared to the values of 9 MPa and 12% shown with the neat PLA electrospun fibers, the presence of mCNT-PLA not only strengthens but also toughens the PLA-based electrospun fibers.