Abstract: Biomass-based composites offer several advantages, including resource renewability, low environmental impact, and, in certain systems, biodegradability, thereby demonstrating considerable application potential in packaging, construction, transportation, and biomedical fields. However, substantial polarity differences between biomass-derived components and polymer matrices, limited interfacial load-transfer efficiency, and the inherent brittleness of certain polymer matrices frequently result in inadequate impact resistance and low fracture toughness. Focusing primarily on natural-fiber- and nanocellulose-reinforced composites, this review systematically summarizes their constituent systems, interfacial characteristics, and representative toughening mechanisms. Particular attention is given to interfacial debonding, fiber pull-out, fiber bridging, crack deflection, and crack branching. Furthermore, the mechanisms, applicable conditions, and limitations of various toughening strategies are comparatively evaluated at three levels: fiber-surface and interphase modification, matrix-phase toughening, and multiphase and multiscale structural design. The strategies discussed include physical modification, chemical modification, nanoparticle reinforcement, flexible-polymer blending, bioinspired layered structures, core–shell structures, and interpenetrating polymer networks. The analysis indicates that effective toughening does not rely solely on increasing interfacial bonding strength; rather, it requires a balance between interfacial load transfer and energy-dissipation processes, including debonding, interfacial sliding, and fiber pull-out. Accordingly, composite structures with appropriate interfacial fracture resistance and hierarchical energy-dissipation capabilities should be constructed. This review provides a theoretical basis and technical reference for the design of high-strength and high-toughness biomass-based composites.