A design of experiments was conducted following a full factorial approach to develop a plastic biocomposite that utilized increased sustainable content, through the inclusion of chain extender, recycled poly(ethylene terephthalate) (RPET) and biocarbon in a toughened-PET matrix. Statistical evaluation of the mechanical properties of the extrusion-injection molded materials were analyzed for their main effects and factor interactions. Enhancement in the tensile strength and Young’s modulus occurred with impact strength elevated to 246 J·m-1, in combination of biocarbon and chain extender, demonstrating their synergistic interaction in the biocomposite system. It was observed that the morphology and mechanical variation was predominantly affected by the biocarbon and chain extender in the formulations, while RPET did not substantially alter the performance compared to its virgin counterpart. Optimization of the biocomposite for good stiffness-toughness balance led to a sample with 28% sustainable content, having 25 wt.% RPET substitution of PET, 10 wt.% biocarbon and 1 phr of chain extender. ACKNOWLEDGEMENTS The financial support from Ontario Research Fund, Research Excellence Program; Round-7 (ORF-RE07) from the Ontario Ministry of Research, Innovation and Science (MRIS) (Project # 052644, 052665); and the Natural Sciences and Engineering Research Council (NSERC), Canada Discovery Grants (Project # 401111 and 400320) to carry out this research is gratefully acknowledged.