A membrane-electrode assembly for a mixed reactant fuel cell and a mixed reactant fuel cell system including the same. In one embodiment of the present invention, a membrane-electrode assembly for a mixed reactant fuel cell includes an anode catalyst layer, a cathode catalyst layer, a polymer electrolyte membrane disposed between the anode catalyst layer and the cathode catalyst layer, an electrode substrate disposed on at least one of the anode catalyst layer or the cathode catalyst layer, and an oxidant supply path penetrating the polymer electrolyte membrane, the anode catalyst layer, the cathode catalyst layer, and the electrode substrate and adapted to supply an oxidant.