The growing demand for environmentally sustainable and biodegradable materials has intensified interest in alternative solutions for thermal insulation. This study explores the development of composite materials using mango seed shell biochar (MSSB) and soy protein isolate (SPI) as a biodegradable matrix-filler system. Mango seed shells, an abundant agro-industrial waste, were subjected to pyrolysis at 500 degrees C for 2 hours to produce biochar. The resulting MSSB was incorporated into SPI with glycerol as a plasticizer to fabricate composite sheets containing 10%, 20%, and 30% biochar by weight Thermal conductivity tests showed that increasing MSSB content led to a notable reduction in thermal conductivity, with the 30% MSSB composite achieving a value of 0.035 W/mK-comparable to commercial synthetic foams such as expanded polystyrene. Mechanical analysis revealed a tradeoff between tensile and compressive properties. While tensile strength decreased from 1.8 MPa for pure SPI to 0.7 MPa at 30% MSSB, compressive strength improved with increasing biochar content, peaking at 1.5 MPa.Biodegradability was evaluated through an 8-week soil burial test, which demonstrated accelerated degradation in composites with higher MSSB content, reaching up to 55% weight loss at 30% loading. These findings highlight the potential of MSSB-SPI composites as eco-friendly insulation materials suitable for green building and packaging applications. Future work will focus on mechanical property enhancement to expand the material's structural utility.