In this study, we analyze the change in the thermoelectric properties of a Mg_x(Si, Sn) system upon varying nominal Mg content x (x = 1.8, 1.9, 2.1, 2.2, 2.3). We find that Mg deficiency induces multiple point defects, such as Mg vacancies and anion anti-site defects, as well as phase separation of crystal structure, both of which alter the band structure. Systematic investigations are followed to elucidate how thermoelectric properties of a Mg_x(Si, Sn) system are influeced by the degree of Mg deficiency. Temperature-dependent measurements of the Seebeck coefficient and the Hall coefficient indicate that Mg deficiency induces p-type behavior; this is confirmed by an analysis of the thermoelectric transport properties by using a single parabolic band model at low temperature. The p-type conduction in the Mg-deficient system suggests that Mg vacancies are the predominant defects. However, the efficiency of p-type doping is two to three orders of magnitude lower (∼10¹⁸ cm⁻³) than expected when considering only Mg vacancies as the origin of charge carriers. Density functional theory calculations suggest that the anion anti-site defect on the Mg site is attributable for the low measured carrier concentration and low doping efficiency. Moreover, the energy band structure of the Mg-deficient Mg_x(Si, Sn) system is shown to undergo significant deformation and allow the formation of an impurity band at the X point due to the Mg-deficient conditions, enabling the system to have a relatively anion-rich condition. Furthermore, Mg deficiency minimizes the thermal conductivity to 1–1.5 W/m∙K at room temperature owing to an early bipolar contribution and separated structural phases. This study provides the Mg deficiency as a new control parameter that can be utilized to enhance the p-type thermoelectric performance of Mg silicides.