As the consumption of coffee increases worldwide, the amount of spent coffee grounds (SCG) is gradually increasing every year. Some of these grounds are recycled for composting, but most are discarded, which causes widespread financial and social costs. We developed a bio-based plastic pellet by blending polypropylene (PP) with waste biomass SCG to convert it into a sustainable, recyclable eco-friendly material. It was confirmed that extrusion compounding for SCG/PP composite pellets and injection molding with good formability are possible. To evaluate the formability of the composite pellets, American Society for Testing and Materials (ASTM) test specimens were prepared for evaluating mechanical properties by injection molding. As a result of the measurement of the test samples, the mechanical properties of SCG/PP composite pellets were generally lowered as the SCG content increased. However, the impact strength of SCG/PP composite based on the HOMO-PP matrix improved as the SCG content increased. In addition, Young’s modulus of SCG/PP increased as the SCG content increased. In the future, this study will be applied to manufacture of products that requires non-toxic products, such as disposable products and food containers, realizing commercialization of eco-friendly products and thereby replacing finite petroleum resources and practicing resource circulation and environmental protection.