This study evaluates bio-stabilised cob’s structural and thermal performance for sustainable buildings. Thirty-three cob mixes were analysed: subsoil, water, and natural fibres—hempshiv and barley straw—with fibre contents ranging from 1% to 10% and water contents ranging from 20% to 30% by volume. Unstabilised samples of subsoil and water served as baselines. X-ray diffraction revealed the subsoil contained 70% quartz, while the fibres were predominantly cellulose. Scanning electron microscopy showed the subsoil had a compact structure with 20.34% porosity, whereas hempshiv and barley straw were more porous, with 29.94% and 42.78%, respectively. Adding fibres significantly reduced thermal conductivity compared to unstabilised mixes. The lowest thermal conductivity was 0.132 W/m·K for a mix with 7% barley straw and 30% water, while the highest was 0.378 W/m·K for the unstabilised mix with 20% water. Higher densities correlated with higher thermal conductivity. Compressive strength decreased with increased fibre content, with hempshiv causing a smaller reduction than barley straw. The highest compressive strength was 1.719 MPa for a mix with 5% hempshiv and 25% water; the lowest was 0.785 MPa for a mix with 7% barley straw and 25% water. Volumetric shrinkage increased with higher fibre and lower water contents. The mix with 3% hempshiv and 30% water content demonstrated optimal performance: thermal conductivity of 0.163 W/m·K, compressive strength of 1.454 MPa, density of 1,676 kg/m3, and volumetric shrinkage of 16.4%. This research highlights the importance of analysing cob’s constituents to optimise its properties, supporting cob’s potential as a sustainable construction material for decarbonising the built environment.