Magnesium Silicide Stannide [Mg2(Si,Sn)]-based materials are known to be an important class of thermoelectric materials integrating the earth-abundant and non-toxic elements. In this paper, we present the electrical and thermoelectric performances of Mg_2-δSi_0.35-xSn_0.65Ge_x (x = 0, 0.05 and δ= 0–0.04) alloys. The alloys have been prepared by mechanical alloying along with vacuum hotpressing technique. The alloys Mg_1.96Si_0.3Sn_0.65Ge_0.05 and Mg_1.98Si_0.3Sn_0.65Ge_0.05 possess low electrical and thermal conductivity, high Seebeck, and significantly high power factor in comparison to the parent Mg₂Si_0.35Sn_0.65 alloy. The substitution of Ge at the Si site serves two purposes. First, it reduced the bipolar effect due to the enlargement of the band gap and subsequently reduced the lattice thermal conductivity. Furthermore, the creation of Mg vacancy has contributed to the enhancement of phonon scattering at the grain boundaries, which in turn enhanced the Seebeck coefficient, reduced the electrical, and thermal conductivity. The synergetic confluence of improved power factor and low thermal conductivity in Mg_1.98Si_0.3Sn_0.65Ge_0.05 resulted in the highest ZT value of 0.08 at ~ 523 K, which is ~ 73% higher than the ZT value (~ 0.02) of the parent Mg₂Si_0.35Sn_0.65 alloy.