Waste polyester fibers (WPFs) were used to prepare a range of high-density polyethylene (HDPE) composites with maleic anhydride-grafted polyethylene (PE-g-MA) as compatibilizer, and these were characterized. The objective of the research was to enhance the mechanical properties of HDPE and to encourage the use of plastic and textile waste. The composite specimens were made using single-screw extrusion, and tensile, flexural, impact, differential scanning calorimetry (DSC), density, porosity, and water absorption were measured. A systematic evaluation of fiber orientation and recycled HDPE (r-HDPE) content was made. The results showed that the fiber orientation had a significant influence on the stress transfer and failure mechanisms. The mesh-reinforced composite configuration had the best overall performance with tensile strength of 52 MPa and impact strength of 487.8 J/m, while the aligned fiber configuration had the highest flexural strength of 104.7 MPa. PE-g-MA was added, which increased the adhesion between the fiber and the matrix and decreased the void formation. With the gradual incorporation of r-HDPE, the mechanical properties were gradually reduced because of the degradation of the polymer and reduction in interfacial interactions, but the composites with up to 15 wt.% r-HDPE still performed well. The results show that the use of polyester fiber-reinforced HDPE waste composite represents a viable approach to producing high-quality engineering materials with high recycled content and improved mechanical properties.