This study investigated the mechanical performance of glass fiber-reinforced epoxy composites incorporating varying percentages of tea waste filler, a globally abundant lignocellulosic by-product, with the objective of evaluating its influence on their tensile and flexural properties and providing insights into its potential as a sustainable filler. Unfilled composites (Category A) exhibited the highest tensile strength (298.69 MPa) and elastic modulus (4928.67 MPa), attributed to optimal fiber-matrix bonding. However, composites with 2% tea filler (Category D) demonstrated the highest toughness (12468.91 kJ/m³) and energy absorption in tensile tests, highlighting the filler’s ability to dissipate energy during deformation. Similar trends were observed in flexural tests; while unfilled composites achieved the highest flexural strength (349.43 MPa) , the addition of 0.5% filler (Category B) enhanced stiffness (elastic modulus: 19230.5 MPa). Higher filler contents generally reduced strength due to particle agglomeration and weak interfacial bonding, but significantly improved toughness and energy absorption under flexural loading. The study underscores the potential of tea waste as a sustainable filler to enhance toughness and energy absorption in polymer composites, making them suitable for applications in industries requiring lightweight and durable materials. Future research should explore advanced filler modification techniques, such as silane treatment, to optimize strength and stiffness by improving filler dispersion and matrix bonding, and assess performance under long-term environmental conditions.