This study presents a high-performance magnesium-based plastic semiconductor, Mg₃Sb₂₋ₓBiₓ, which exhibits exceptional thermoelectric performance and plasticity at room temperature. The material demonstrates a high figure of merit (zT) of ~0.72 and a large strain of ~43%, surpassing conventional brittle thermoelectric materials like Bi₂(Se,Te)₃ and plastic organic semiconductors. The high plasticity is attributed to a dense dislocation network and persistent Mg-Sb/Bi bonds during deformation. The material can be easily processed into micro-scale dimensions, enabling the fabrication of both in-plane and out-of-plane flexible thermoelectric modules with promising power density. The combination of high thermoelectric performance and plasticity makes Mg₃Sb₂₋ₓBiₓ a promising candidate for flexible electronics. The study also reveals the underlying mechanisms of plastic deformation in Mg₃Sb₂ and Mg₃Bi₂, including small generalized stacking fault energy (GSFE) and large cleavage energy (CE), which facilitate atomic layer slipping. The material's high toughness and good machinability allow it to be diced into small dimensions without damage, enabling the development of micro-scale thermoelectric modules. The study highlights the potential of Mg₃Sb₂₋ₓBiₓ for flexible thermoelectric applications and suggests that further optimization of interface properties and development of high-performance p-type Mg-based thermoelectric materials could significantly enhance its performance.This study presents a high-performance magnesium-based plastic semiconductor, Mg₃Sb₂₋ₓBiₓ, which exhibits exceptional thermoelectric performance and plasticity at room temperature. The material demonstrates a high figure of merit (zT) of ~0.72 and a large strain of ~43%, surpassing conventional brittle thermoelectric materials like Bi₂(Se,Te)₃ and plastic organic semiconductors. The high plasticity is attributed to a dense dislocation network and persistent Mg-Sb/Bi bonds during deformation. The material can be easily processed into micro-scale dimensions, enabling the fabrication of both in-plane and out-of-plane flexible thermoelectric modules with promising power density. The combination of high thermoelectric performance and plasticity makes Mg₃Sb₂₋ₓBiₓ a promising candidate for flexible electronics. The study also reveals the underlying mechanisms of plastic deformation in Mg₃Sb₂ and Mg₃Bi₂, including small generalized stacking fault energy (GSFE) and large cleavage energy (CE), which facilitate atomic layer slipping. The material's high toughness and good machinability allow it to be diced into small dimensions without damage, enabling the development of micro-scale thermoelectric modules. The study highlights the potential of Mg₃Sb₂₋ₓBiₓ for flexible thermoelectric applications and suggests that further optimization of interface properties and development of high-performance p-type Mg-based thermoelectric materials could significantly enhance its performance.