Rising agricultural and plastic wastes pose a severe environmental problem, as the plastic and agricultural wastes need to be valorized in sustainable ways. Recycled sources of low-density polyethylene (LDPE) were used in this experiment as a matrix, whereas rice husk powder, groundnut shell powder, and recycled high-density polyethylene (HDPE) flakes were used as fillers. Agricultural fillers were treated using alkali (NaOH) to offer improvement in adhesion as well as dispersion in the polymer matrix and the interfacial adhesion. The process of making composites was done through blending using mechanics, after which they were subjected to compression molding. Detailed characterization was performed in order to determine the mechanical, thermal, and morphological behavior of the developed composites. By using the test/mechanical test by ASTM specifications, it was noted that the alkali-treated fillers resulted in substantially better tensile strength, flexural strength, impact resistance, and Shore D hardness as compared to the untreated group. Stability to thermal and crystallinity tests was carried out by thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), respectively, where results showed that composites treated had increased decomposition temperatures and crystallization behavior. These results were also supported by scanning electron microscopy (SEM) that showed homogeneous dispersion of fillers and increased bonding between the filler and the sample as a result of alkali treatment. WAB, measured according to ASTM D570, indicated a lower uptake on treated composites, and this indicated the existence of a more compatible filler-to-matrix system. Also, an upfront life cycle assessment (LCA) showed probable decreases in carbon footprint and resource utilization because of the incorporation of waste-based fillers. This paper finds that the hybrid filler systems, made of agricultural and plastic waste, when treated, can be utilized as a simple modality to achieve a high-performance, sustainable polymer composite performance that can be used for structural and packaging purposes.