Abstract <p>Waste edible oil is dumped in massive quantities all around the world, posing a severe threat to the environment. The present research deals with the cost-effective, facile, environmentally benign method of synthesizing carbon nano-onions (CNOs) using waste flaxseed oil for high-performance supercapacitor device. CNOs are potential candidates for supercapacitor electrodes owing to their distinct 0-D configuration, improved electrical conductivity, and comparatively easy dispersion. In this study, CNOs are synthesized by pyrolyzing waste flaxseed oil via a traditional method commonly employed in Indian demography to prepare Kajal. The KOH activation of the as-prepared CNOs incorporated carboxyl (−COOH) functionalities, rendering them hydrophilic, as confirmed by Fourier transform infrared spectra. These activated CNOs are characterized using X-ray diffraction and Raman spectra, which exhibit the presence of graphitic carbon with sp² hybridization in the formed activated carbon as the major component. Transmission and scanning electron microscopy reveal that activated CNOs show a distinctive appearance having centered nanographene shells of an interconnected network that are beneficial for charge transportation. The electrochemical analysis is carried out using three-electrode cell assembly in 6 M KOH electrolyte, which demonstrates that the flaxseed oil-derived activated CNOs possess a specific capacitance of 745.9 F g⁻¹ at a current density of 2 A g⁻¹. Furthermore, it is observed that the device presents good cycling stability and retaining approximately 89% of charge storage characteristics over 5000 charge–discharge cycles with maximal energy density of 22.5 Wh kg⁻¹&#xa0;and power density of 2102 W kg⁻¹. The conversion of used (waste) flaxseed oil into activated carbon to be used as an electrode material for supercapacitor applications has led to the sustainable development of energy.</p> Graphical abstract <p></p>