Waste eggshells were used as a raw material to produce high surface area biocalcium carbonate nanoparticles using a combination of mechanochemical and ultrasonic irradiation techniques. High resolution transmission electron microscopy (HR-TEM) and X-ray analysis showed that the synthesis process was effective and yields only CaCO3 nanoparticles with high porosity. The bionanocomposites were prepared by infusing three different weight percentages (1%, 2%, and 3%) of the calcium carbonate nanoparticles (CaCO3) in Bioplast GS 2189 thermoplastic polymer. Thermal analyses indicated that the 2% bionanocomposites are thermally more stable (by 15 °C) than the corresponding neat systems. Mechanical test results of 2% bionanocomposite showed an approximately 35.3% and 30.5% increase in the flexural strength and modulus, respectively, over the pristine Bioplast GS 2189 polymer.