Green composites of poly(lactic acid) (PLA) and waste cellulose fibers (WCF) were produced by using a facile technique comprising high-shear mixing within relatively short processing times that facilitates the ease of processing of such materials and ensures the homogeneous dispersion of such fibers in thermoplastics due to shear rates as high as 5200 rpm. Key parameters, such as optimal concentrations, homogeneous dispersion, direct and indirect mechanical contributions of the fibers, interfacial interactions, and crystallinity of the PLA matrix, were examined for the sustainable production of PLA/WCF green composites with enhanced stiffness, strength, toughness, and impact resistance. Briefly, around one-fold, 50%, and 20% increase in the elastic modulus, tensile strength, and impact strength of PLA, respectively, were achieved by the addition of 30 wt % WCF. In addition, an 87% increase in the impact strength of PLA was also achieved by the incorporation of 5 wt % WCF.