In natural bio-based composites are emerging as viable alternatives to synthetic fiber composites, with the incorporation of eggshell bio-filler in polymer composites being explored for environmental conservation. Constructing sustainable composite materials using natural biopolymers holds immense promise due to their biodegradable, biocompatible, and renewable properties. In this study, advanced sustainable composite materials were developed by incorporating eggshell as a green filler within composites made from a matrix of banana peel and potato starch. The optimum mechanical strength (9.88 Mpa), optimum moisture uptake (13.11), optimum biodegradation (83.38 ± 0.012) was found at 20% plasticizer and 60% eggshell. The uniform filler size and strong interfacial hydrogen bonding facilitated the successful integration of the eggshell powder (ESP) filler. Composites were created with 2–60% ESP and 5–20% plasticizer, and the mechanical and biodegradability properties were investigated. Composites with 30% ESP exhibited significant improvements in mechanical characteristics, and the prepared biocomposites showed enhanced mechanical properties, improved thermal stability, and stronger matrix-filler interactions.