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In this study, material characterization and a cradle-to-gate life cycle assessment methodology are applied to examine how the addition of biochar as a filler in recycled plastics (rHDPE) influences material properties and environmental burdens. Environmental impacts for these composites are then compared to biochar fillers in virgin HDPE and bio-based polylactic acid (PLA) and polyhydroxybutyrate (PHB). At 40% biochar addition to rHDPE, biochar increased the tensile strength by 45%, stiffness by 126% and flexural storage modulus by 79% but resulted in a more brittle material. Biochar addition detrimentally affects the thermal degradation of both PLA and PHB, resulting in reduced mechanical properties. Increased biochar loading linearly reduced the global warming potential of all plastics relative to neat plastic by up to 3.3 kg CO2 eq. per kg of composite, with a dual benefit of reducing the amount of plastic used and creating a net-uptake of carbon in the biochar. Similarly, biochar decreased the fossil fuel depletion of PLA, PHB, and HDPE, but additional transportation-related emissions and the low fossil fuel depletion for neat rHDPE caused biochar addition to increase fossil fuel depletion for rHDPE. While changes in material application and end-of-life were outside the scope of this study, biochar is expected to provide further end-of-life benefits to the biodegradable PHB or PLA. Importantly, this study demonstrates that biochar can be applied to produce carbon-neutral composite materials when added to a wide variety of plastics and demonstrates the potential of biochar to reduce the environmental impacts of plastic materials.