Mg2XIV (XIV = Si, Ge, Sn) compounds are semiconductors and their solid solutions are believed to be promising mid-temperature thermoelectric materials. By contrast, Mg2Pb is a metal and few studies have been conducted to investigate the thermoelectric properties of Mg2Si-Mg2Pb solid solutions. Here, we present a theoretical study exploring whether Mg2Pb-Mg2Si solid solutions can be used as thermoelectric materials or not. We firstly constructed several Mg2Si1-xPbx (0 ≤ x ≤ 1) structures and calculated their electronic structures. It is suggested that Mg2Si1-xPbx are potential thermoelectric semiconductors in the range of 0 ≤ x ≤ 0.25. We then explicitly computed the electron relaxation time and both the electronic and lattice thermal conductivities of Mg2Si1-xPbx (0 ≤ x ≤ 0.25) and studied the effect of Pb concentration on the Seebeck coefficient, electrical conductivity, thermal conductivity, and thermoelectric figure of merit (ZT). At low Pb concentration (x = 1/16), the ZT of the Mg2Si1-xPbx solid solutions (up to 0.67 at 900 K) reaches a maximum and is much higher than that of Mg2Si.