Silver (Ag) doped Mg2Si1-xGex (x=0.1 to 0.4) samples were fabricated using a plasma activated sintering (PAS) method. The doping concentration of Ag was varied from 1 to 5 at.%. Undoped Mg2Si1-xGex exhibits n-type conductivity due to residual impurities in the Mg source material used and unintentionally process-induced impurities. The observed unstable behavior of the Seebeck coefficient of Ag-doped p-type Mg2Si1-xGex (x ≤ 0.3) in the region of 550 to 650 K, exhibiting a considerable drop in the value and occasional conduction type conversion, was correlated with the specific contaminants. For x∼0.4, the observed Seebeck coefficient varied from 0.2 mV/K at 823 K to 0.4 mV/K at room temperature, with no remarkable drop in the value with increasing temperature. An estimated ZT value of 5 at.% Ag doped Mg2Si0.6Ge0.4 was 0.18 at 844 K. It was found that both specific residual impurities and process-induced impurities affected the characteristics of the Seebeck coefficient of Mg2Si1-xGex.