Renewable energy and environmental preservation are two grand challenges in our society today. To address these two challenges, there is an increasing demand for energy storage devices made of green and biodegradable materials. State-of-the-art plant-based electrodes have problems of poor electrochemical performance, low reliability, and high manufacturing cost that pose major limitations in their use in flexible supercapacitors. In this research, a novel microwave irradiation synthesis is used to produce a high-performing electro-active lignin-based biomaterial. MnO2 particles are deposited on these lignin-based materials to impart pseudo-capacitance property. These electro-active materials were coated on an Al substrate and used as an anode with an AC-based cathode. A quasi-solid-state supercapacitor was assembled using a polymer-based gel electrolyte of PVA/H3PO4. SEM was performed to study morphology, porosity, and polydispersity of the lignin-based matrix. Cyclic voltammetry (CV) was employed to study the polarization resistance of the system. The cyclic charge-discharge (CCD) was performed to observe cyclic performance. The assembled supercapacitor exhibited a specific capacitance of 26 mF/g after 500 cycles with capacitance retention of 87% at 0.1 A/g. This work provides new insights into the synthesis of low-cost and scalable plant-based flexible supercapacitors.