Plastics are ubiquitous in modern society, offering desirable material properties, but they are primarily derived from fossil fuels, leading to significant environmental concerns such as long-term pollution. As sustainable alternatives, bioplastics and plastic bio composites have emerged as promising solutions by potentially increasing degradability or reducing the overall amount of plastic entering the environment. This research investigates the use of shiitake mushroom powder as a filler in polylactic acid (PLA) to reduce the volume of pure PLA used and improve its mechanical properties while exploring scalable manufacturing methods for plastic alternatives. The composite was produced using twin-screw extrusion (TSE) and blended with PLA at concentrations of 5%, 10%, 15%, and 20% shiitake powder by volume. Type A1 tensile, and unnotched type impact specimens were produced using injection molding using ISO 527-2 and ISO 180 standards. Tensile and impact testing revealed that PLA-shiitake composites exhibited enhanced mechanical properties compared to pure PLA, with the highest ultimate tensile strength (50.8 MPa) observed at 5% shiitake concentration and the highest peak impact strength (40,896.2 J/m²) at 20%. Scanning electron microscopy (SEM) confirmed a homogeneous distribution of shiitake powder within the PLA matrix. These results suggest that TSE and injection molding used in conjunction are effective approaches for producing PLA bio composites and presents a scalable and cost-effective approach for manufacturing sustainable plastic alternatives.