This work reports a simple and cost-effective synthesis path to fabricate manganese dioxide nanostructures with controlled morphologies. Herein, we have used oleylamine as a reducing, surfactant, and structure directing agent. The treatment of MnO2 precursor with the varying concentration of oleylamine led to the formation of different morphologies of MnO2 (nanorods, nanoflowers and three-dimensional MnO2 spheres). The as-synthesized nanostructures were characterized by various techniques and their electrochemical performances were studied. When used as an electrode material for supercapacitors, 3D-MnO2 exhibited a higher specific capacitance of 450 Fg(-1) and admirable cyclic stability (similar to 91.7%) after 5000 cycles, which were higher than that of the other formulations. Moreover, an asymmetric supercapacitor was assembled by using as-prepared 3D-MnO2 and nitrogen-doped graphene hydrogels as the positive and negative electrodes, respectively. As-fabricated asymmetric device delivered specific capacitance of 49.44 Fg(-1) at 1 Ag-1 an outstanding energy density of 20 Whkg(-1) at 857.14 Wkg(-1), and retained similar to 93.15% stability after 5000 cycles. The obtained results showed that the as-synthesized 3D-MnO2 can be considered as the positive electrode material in supercapacitors.