Abstract: Carbon dots (CDs), as a new class of zero-dimensional carbon-based nanomaterials, have attracted considerable attention for enhancing the mechanical properties of materials owing to their small size, abundant surface functional groups, excellent dispersibility, and tunable interfacial interactions. This review systematically summarizes the recent advances in the mechanical reinforcement of materials by CDs. First, the effects of the structural types, surface chemical characteristics, dispersion state, and preparation methods of carbon dots on their mechanical reinforcement performance are introduced. Subsequently, the research progress and reinforcement behaviors of CDs in various matrix systems, including natural polymers, synthetic polymers, hydrogels, and inorganic/hybrid materials, are comprehensively discussed. Existing studies demonstrate that CDs can significantly improve the strength, toughness, and structural stability of materials, and their reinforcing effects are closely related to the structural characteristics of CDs and the properties of the matrix. Furthermore, the mechanisms underlying the mechanical reinforcement by CDs were summarized and classified into four categories: interfacial strengthening, microstructural regulation, energy-dissipation optimization, and synergistic reinforcement. Finally, the challenges associated with CD-reinforced composites, such as the establishment of structure–property relationships, large-scale production, and engineering applications, are discussed, and future research directions are highlighted. This review provides theoretical guidance and valuable insights for the design and application of high-performance CD-reinforced composites.