<p>The rapid advancement of electrochemical energy storage and conversion technologies has driven significant interest in MXene-based nanocomposites, owing to their high electrical conductivity, tunable surface chemistry, and layered architecture. This review presents a critical and mechanistic analysis of MXene nanocomposites for batteries and fuel cells, focusing on rational design strategies such as interlayer engineering, surface functionalization, and hybridization with metal oxides, sulfides, polymers, and carbon materials to address challenges including oxidation, restacking, and structural instability. A key contribution is the establishment of structure–property–performance relationships, highlighting how surface terminations, interlayer spacing, and heterointerfaces regulate ion transport, charge storage mechanisms, and electrocatalytic activity. Comparative insights across lithium-, sodium-, potassium-, zinc-, and magnesium-ion batteries, as well as fuel cell systems, are provided to identify performance trends and limiting factors. The integration of experimental studies with theoretical approaches, including density functional theory and emerging machine learning techniques, is discussed to elucidate ion adsorption, diffusion kinetics, and degradation pathways. Finally, critical challenges related to scalability, environmental stability, and practical implementation are evaluated, and future directions are proposed, emphasizing interface engineering, controlled surface chemistry, and device-level validation. This review provides a unified framework for the rational design of MXene-based nanocomposites in next-generation electrochemical energy systems.</p>