The preparation of an environmentally benign, inexpensive supercapacitor electrode material from waste biomass, achieving useful carbon electrode production, is challenging. The present work aims to find a process for recycling spent mushroom compost into a useful supercapacitor electrode material. High-surface-area activated carbon derived from spent mushroom compost (SMC) is used as a catalyst electrode in a solid-state electrochemical double-layer capacitor (SSEDLC). The SMC is thermally activated at three different temperatures (SMC-700, SMC-800, and SMC-900) to achieve the optimal carbon content and activation temperature for improving electrode performance based on the surface area and pore structure. The SMC-derived nanostructured carbon exhibited a high specific surface area of 690 m² g⁻¹ in the case of SMC-800 upon activation, which strongly influences the specific and areal capacitance of the fabricated SSEDLC. Interestingly, the use of the prepared PVA/H₂SO₄ gel electrolyte showed a promising trend, with considerable capacitance retention of up to 90% over 3000 cycles obtained with the material optimized at 800 °C (SMC-800).