Crystalline hausmannite nanocrystals/reduced graphene oxide composites (denoted as Mn3O4/RGO) are synthesized by means of the microwave-assisted hydrothermal synthesis (MAHS) route for the application of symmetric and asymmetric supercapacitors. Effects of the Mn3O4 content on the microstructure and electrochemical performances of nanocomposites are systematically investigated. The morphology and crystalline structure of composites are examined by scanning electron microscopic (SEM) and X-ray diffraction (XRD) analyses. Thermogravimetric analysis (TGA) is used to determine the mass content of RGO in the Mn3O4/RGO composites. The results indicate that the composite with about 82.4 weight percents (wt%) Mn3O4 (denoted as GMn80) shows the largest total specific capacitance (∼193 F g−1), which is employed for the tests of supercapacitors of both symmetric and asymmetric types. An aqueous asymmetric supercapacitor, consisting of a GMn80 cathode and an N-doped RGO anode (reduced from a GO-NH4OH solution through the MAHS method), with a cell voltage of 2 V is demonstrated in this work, which shows the device energy and power densities of 11.11 Wh kg−1 and 23.5 kW kg−1, respectively.