Achieving high-performance cellulose biocomposites requires precise nanostructural control to optimize interfacial interactions, mechanical strength, and functional properties. We explored diverse strategies for enhancing cellulose-based composites through interfacial engineering, cross-linking, and hierarchical structuring. Carboxymethyl cellulose (CMC) functionalized with hydrophobic quaternary ammonium ions (QAs) was employed to improve resin impregnation in holocellulose fiber (HF) networks, resulting in optically transparent and mechanically robust acrylic composites. Similarly, calcium-ion cross-linking between carboxylated cellulose nanofibrils (TCNF) and alginate formed an interpenetrating double network, yielding exceptional stiffness and tensile strength, with promising applications as sustainable barrier coatings. Additionally, a hybrid paper featuring cellulose microgels and nanofibers significantly enhanced fiber-to-fiber adhesion, achieving ultra-high strength and excellent wet-state performance. Lastly, a facile rehydration approach facilitated silk fibroin infiltration into CNF films, enhancing their mechanical properties and biocompatibility for biomedical applications. Collectively, these approaches demonstrate how tailored nanostructural control in cellulose composites enables multifunctional, high-performance materials suitable for optoelectronics, packaging, and biomedical applications.