Magnesium–gallium–mercury alloy is one of the new developed anode materials for seawater activated batteries. The potentiodynamic polarization, galvanostatic discharge and electrochemical impedance spectroscopy of Mg–4%Ga–2%Hg alloy in seawater are studied and compared with commercial AZ31 and AP65 alloys in this study. The results show that Mg–4%Ga–2%Hg alloy exhibits different discharge behaviors in as-cast, homogenizing, rolling and annealing conditions. The annealing Mg–4%Ga–2%Hg sheet obtains the most negative corrosion potentials in different current densities. And the Mg–4%Ga–2%Hg alloy provides more negative corrosion potentials than AZ31 and AP65 alloys. EIS studies reveal that the Mg–Ga–Hg alloy/seawater interfacial process is determined by an activation-controlled reaction. The Mg3Hg and Mg21Ga5Hg3 phases improve the electrochemical properties of Mg–4%Ga–2%Hg alloy. The assembled prototype battery with Mg–4%Ga–2%Hg alloy as anode and CuCl as cathodes exhibits a satisfactory discharge performance because of the advantages in discharge characterizations and microstructure of the Mg–4%Ga–2%Hg alloy.