The recently proposed hybrid Mg-Li ion battery technology delivers good reaction kinetics and excellent safety features because it integrates the advantages of the Mg metal anode and the Li+ ion intercalation/conversion electrodes. The inexpensive and resource-abundant FeS is a suitable candidate as a cathode material due to its medium redox potential and high theoretical capacity; however, a conventional FeS cathode suffers from low electronic conductivity, low coulombic efficiency and poor cycling performance. Here, in this work, to overcome these drawbacks, a free-standing membrane electrode with FeS nanoparticles embedded in carbon nanofibers was designed and prepared via a facile electrospinning method. Additionally, because of the absence of inactive materials, such as the current collector and binder, the energy density was enhanced compared with that in the conventional batteries. The hybrid Mg-Li ion battery fabricated in this study can deliver a maximum energy density of ∼300 Wh kg−1, which is superior to that of most conventional Mg ion batteries. An outstanding cyclic performance for up to 800 cycles with high coulombic efficiencies above 97% was demonstrated, and a specific capacity above 200 mAh g−1 was retained with a current density of 257 mA g−1.