We report the concept of stiff, strong, and lightweight multifunctional composites that can perform as a structural composite supercapacitor. The self-assembly of Manganese Dioxide (MnO2) nanowhiskers decorated carbon fiber (MO-CF fiber) composites enabled improvement in surface area, electrical conductivity, and a high number of active sites of the carbon fiber electrodes, thereby, exhibiting excellent charge storage capacity for the composite supercapacitors. The MO-CF-integrated composites achieved a specific capacitance of 1.1 F g−1, which is a 120-fold increase compared with neat carbon fiber–based supercapacitors. Moreover, the composite supercapacitor device showed an energy density of 2.45 W h kg−1. The fabricated device also achieved a capacitance of 1.71 F g−1 and an energy density of 3.8 Wh kg−1 in a 3 M NaCl solution. With an exceptional increase in its electrochemical properties, the MO-CF-based composite also maintained its mechanical properties, exhibiting an increase in tensile strength up to 123.6 MPa. Additionally, we demonstrated the feasibility of using a MO-CF composite as a structural supercapacitor by illuminating an LED.