This article focuses on the development of starch-based biodegradable polymer composites reinforced by agricultural byproducts. Maize hull and sugarcane bagasse were used as reinforcements in maize starch, which was plasticized by glycerol and maltitol. Compounds with different compositions were produced by compounding in a twin screw extruder. The mechanical (tensile-, three point bending-, and Charpy-impact) properties and shrinkage were determined on injection-molded specimens and compared with those of commercial HDPE. The mechanical properties of thermoplastic starch increased with the addition of both maze hull and sugarcane bagasse, but only the latter ensured comparable results with reference HDPE. This was traced to differences in the interfacial adhesion between filler and matrix as observed by scanning electron microscopy. Distance spacers for pizza boxes, usually made from HDPE, were injection molded from these biodegradable composites and subjected biodegradation in soil. The related results indicated that this biodegradable spacer is a good and environmentally benign alternative for the HDPE counterpart.