Mechanically robust composite nanofibers (NFs) with enhanced magnetic properties were made from polyvinylidene fluoride (PVDF) and Fe3O4 nanoparticles (NPs) by an electrospinning method. At up to 11.3 wt %, Fe3O4 NPs were embedded randomly in the PVDF NFs, but when the content exceeded 17 wt %, the NPs aggregated on the NF surfaces. Magnetization of the composite NFs consistently increased with the increasing Fe3O4 NP content. The mechanical strength of the Fe3O4 NP/PVDF composite NF was enhanced by a dispersion strengthening mechanism. A triboelectric nanogenerator was made from the composite, which showed enhanced output performance with the Fe3O4 NP content less than 11.3 wt %, but the performance degraded at higher content. These results were attributed to the electret doping effect and surface aggregation of the Fe3O4 NPs on the NFs, respectively.