The increasing international need of energy storage technologies that are sustainable and efficient, has stimulated an extensive-research beyond the traditional batteries. The supercapacitors have become worthy of consideration because of power density and high charge discharge as well as long cycling life. More recently, a new category of two-dimensional transition metal carbides, nitrides, and carbonitrides (MXenes) have gained immense publicity as state-of-the-art electrode LiCoO₂ supercapacitor electrodes. Their exceptional combination of metallic conductivity, hydrophilicity, variable interlayer distance and mechanical flexibility, render them exceptionally appropriated in future flexible and wearable devices. In this review, a detailed study of MXene-based supercapacitors, strategy of synthesis, structure, and energy storage mechanisms, including electrical double-layer capacitance, pseudocapacitance, and hybrid processes are presented. The significant progress of pristine MXenes, surface-engineered MXenes, MXene-carbon composites, MXene-polymer composites, hierarchies are critically assessed, and their electrochemical performance indicators are presented. Moreover, complementary characterization techniques with high resolutions in the study of the MXene structure and surface chemistry, electrochemical activity are overviewed. Although there have been notable improvements, there remain the challenge of instability of oxidation, interlayer restacking, dangerous synthesis pathways, and scalability issues. The outlooks of the future imply the necessity of green synthesis, interlayer-engineering, hybridization with new materials, and AI/ML-based design solutions. All in all, MXenes have immense possibilities to transform the concept of high-performance flexible and sustainable energy storage.