In the present work, we have studied the growth of Mg ultrathin films on the Si(111) surface modified by Pb reconstructions using low-energy electron diffraction and the four-point-probe method. The new binary surface reconstructions √7×√7-(Mg, Pb) and √19×√3- (Mg, Pb) have been observed for the first time. The growth of magnesium layers depends both on the structure of Pb-induced surface reconstruction and on the Mg deposition manner. It is assumed that inclusion of magnesium atoms in the √7×√7 surface structure stabilizes the growth of Mg film which is independent of the manner of deposition in this case. We have investigated surface electrical conductance after the formation of magnesium layers at room temperature until about 9 monolayers of Mg coverage. In addition to the magnesium film Mg(0001), the highest electrical conductance among the presented surface structures has been detected for the Si(111)-6×6-(Pb, Mg) surface phase that consists of the maximum number of metal atoms, both lead and magnesium.