Herein, the Maillard reaction for functionalizing soy protein isolate (SPI) based films using hemicellulose hydrolysate derived from Miscanthus is reported. Heat treatment at 120 °C for 9 h induces covalent crosslinking between protein amino groups and hydrolysate carbonyl groups, resulting in structural densification and pronounced browning associated with the formation of Maillard reaction products. The results of ultraviolet–visible spectroscopy confirmed the accumulation of early- and advanced-stage products of the Maillard reaction, while field-emission scanning electron microscope analysis results exhibited reduced film thickness and a highly compact microstructure. These structural changes significantly improved the tensile strength, Young’s modulus, and toughness without compromising flexibility. Water contact angle measurements, water vapor permeability tests, and swelling ratio analysis demonstrated enhanced hydrophobicity and barrier properties. Analysis of total soluble matter under acidic, neutral, and alkaline conditions confirmed increased pH stability. Furthermore, antioxidant activity tests showed that activity increased as the hydrolysate content increased. Despite the structural reinforcement, the films maintained rapid biodegradation under composting conditions and exhibited no phytotoxicity in seed germination tests. This study highlights Miscanthus-derived hemicellulose hydrolysate as an effective, sustainable modifier for producing multifunctional, biodegradable protein-based films.