In this work, the effects of different fiber loadings on the mechanical properties of the composites at the sub-micron scale were studied through nanoindentation followed by physical characterization. The composites were prepared by incorporating different loadings of wheat straw, corn stalk, and rice husk in polypropylene copolymer using a melt processing method followed by thermal–hydraulic compression technique. Nanoindentation experiments in quasi-continuous stiffness mode were performed on the surfaces of produced composites to study the composites’ elastic modulus, hardness, and creep properties. The obtained results expressed the in-depth study of the micro- and macro-level structure and behavior of particle interactions. The findings demonstrated that observable shifts in composites’ hardness, elastic modulus, and creep rate had occurred. The WS-reinforced biocomposite sheet showed the highest elastic modulus of 1.09 and hardness of 0.11 GPa at 40 wt% loading in comparison to other loadings. An impact strength of 7.55 kJ/m2 was noted for the biocomposite at 40 wt% RH loading. In addition, optical microscopy, Fourier transform infrared spectroscopy, water absorption, thickness swelling, and Vicat softening point studies were conducted on biocomposite sheets to evaluate differences in physical, mechanical, and thermal properties. The outstanding mechanical performance of the newly developed composites makes them suitable for use as a biodegradable packaging material.