The global consumption of plastics is projected to triple, reaching 1,231 million tons by 2060. Managing such vast quantities of plastic waste poses significant environmental challenges. Researchers worldwide have been working on reinforcing plastic with biodegradable materials to address these issues. Between 2002 and 2024, over 2000 studies on bio-plastic composites were conducted across over 85 countries. This study used chicken eggshells, a novel bio-filler material, to reinforce polypropylene (PP), one of the most commonly utilized plastics. Eggshells (Es) were incorporated into PP at varying weight percentages (10%, 20%, and 30%) and processed using injection molding and compression techniques. The effects of Es on the mechanical properties of PP, including tensile strength, tensile modulus, elongation at break, flexural strength, and flexural modulus, were systematically evaluated. Additionally, thermogravimetric analysis (TGA) and its derivative (DTG) were employed to assess the thermal stability of both the filler and the composites. Scanning electron microscopy (SEM) was used to investigate the morphology of Es flakes, the eggshell membrane, and the bonding behavior between Es and PP. The findings revealed that incorporating 30 wt.% Es improved the flexural modulus and tensile modulus of PP by 32% and 12%, respectively, while maintaining the flexural strength of pure PP across all tested Es weight percentages. Thermal analysis demonstrated that Es enhanced the thermal stability and reduced the flammability of PP. SEM analysis confirmed strong bonding between the Es membrane and PP and effective interfacial adhesion between the Es outer layer and PP. These results underscore the potential of eggshells as a sustainable bio-filler for improving polypropylene's mechanical and thermal properties.