The optimized thermoelectric figure of merit (ZT) of Mg2Si0.4Sn0.6 peaks at about 750 K because its relatively narrow band gap results in pronounced bipolar transport at higher temperatures. To suppress the bipolar transport, we have conducted a combined experimental and theoretical investigation of Ge-substitution effects on the band structures and thermoelectric properties of Sb-doped Mg2Si0.4Sn0.6−yGey (y = 0, 0.1, and 0.2) synthesized by solid state reaction. The measured transport properties of these compositions can be interpreted by a triple-parabolic-band model based on first-principle calculation of band structures. The results show that the bipolar conduction in the temperature range up to 800 K was effectively suppressed by Ge substitution that widens the band gap. As a side effect, Ge substitution induces the separation of two otherwise converged conduction bands in Mg2Si0.4Sn0.6, leading to reduced thermoelectric performance at low temperatures. The result of these two competing effects is that Ge-substituted samples achieve the maximum power factor similar to that of Mg2Si0.4Sn0.6, but with an increased peak temperature. In addition, Ge substitution reduces the bipolar thermal conductivity while maintaining the low lattice thermal conductivity of Mg2Si0.4Sn0.6.