Graphene sheets (GNS) have been synthesized using a fast and effective microwave autoclave method in which the MnO2 nanoflakes are coated on the GNS in situ to form a composite material. The structures, compositions, and morphologies of the samples were characterized by X-ray diffraction, Raman spectroscopy, thermogravimetric analysis, and electron microscopy. Application of the catalyst cathodes in Li–O2 batteries was also investigated. Compared with GNS cathode, dramatic improvements in the catalytic performance of the composite cathode have been obtained. This superior performance is attributed to the synergistic benefits from the GNS with three-dimensional structure and the MnO2 nanoflakes that cover them. The GNS not only increase the electrical conductivity of the composite cathode but also offer enough space for the tri-phase reaction and to buffer the volume changes during cycling. Due to their intrinsically high catalytic activity, the MnO2 nanoflakes could efficiently boost the oxygen reduction reaction and oxygen evolution reaction, improving the electrocatalytic performance of the MnO2/GNS composite as cathode for Li–O2 batteries.