Blending polymers with a high concentration of bio-based fillers is one of the solutions that not only reduces a dependency on petroleum-based feedstocks but can also significantly decrease the carbon footprint. In the current study, n-octadecyl isocyanate (ODI) molecules were grafted on lignin particles to render them compatible with a novel copolyamide matrix, which was successfully synthesized through a copolymerization between petroleum- and bio-based monomers. Different concentrations of the surface-modified particles were melt-blended with a low-melting point copolyimide, and the properties of the developed biocomposites being thoroughly studied. The SEM imaging revealed that the surface-modified particles homogeneously dispersed into the polymer matrix for all loading levels without any clear evidence of particle agglomeration, phase separation, or voids formation, proposing excellent compatibility between the components that arose from a successful surface modification process. Furthermore, the mechanical properties of the biocomposites significantly improved. For instance, the yield stress and tensile modulus were enhanced by 50% and 200% at the biocomposite with 50 wt% filler content, without any considerable change in the tensile strain. The dynamic mechanical analysis, as well as the rheology measurements, further confirmed the uniform dispersion of the surface-modified particles and their compatibility with the copolymer matrix, within which the storage modulus considerably improved upon the increase of filler content. Overall, our findings strongly suggest that these newly developed biocomposites with a green content of up to 80% are attractive candidates for substituting petroleum-based plastics for the demanded applications.