Out of 300 million tons of plastics produced globally, only 10% are being recycled. Due to this, there is always a demand for alternative, effective methods to cope with this low recycling rate of plastics. This work demonstrates such an effective technique for recycling high-density polyethylene (HDPE) wastes, incorporating biochar synthesized from postharvest hemp crop residue as fillers. Generally, carbon-based materials are considered to have inert surface properties. However, hemp-derived carbon was successfully functionalized in this work using low-temperature plasma (LTP) treatment in the presence of sulfur hexafluoride (SF 6 ) gas. Changes such as the presence of fluorine functional groups and alterations in the surface morphology, which resulted in surface area enhancement, were observed with the LTP treatment. An X-ray diffraction study of the carbon samples revealed the formation of a new peak at 18.98°, which corresponds to the [002] lattice planes because of the LTP treatment. The presence of fluorine in the LTP-treated carbon samples was confirmed with X-ray photoelectron spectroscopy and Fourier transform infrared spectroscopy. The composite films reinforced with LTP-treated carbon had a significant increase in crystallinity percentage and mechanical properties compared to those of films reinforced with untreated carbon. The HDPE/carbon films loaded with 1 wt % of LTP-treated carbon had superior mechanical and thermal properties compared to the rest of the samples. Compared to HDPE films without any carbon fillers, the HDPE/carbon films loaded with 1 wt % LTP-treated carbon significantly improved elastic modulus and maximum tensile strength by around 3.4 times and 3.7 times, respectively. However, when compared to the neat HDPE film, there was a significant drop in the ability to strain with the LTP-treated, carbon-loaded film samples. The overall crystallinity of HDPE increased by a maximum of 43.8% by the addition of LTP-treated carbon. The LTP treatment was found to be effective in enhancing the interfacial adhesion of carbon and its interaction with the HDPE matrix.