The conductivity in the monoclinic polymorphs of Ln2MoO6 (Ln = Sm, Gd, Dy) oxymolybdates was investigated by theoretical and experimental methods. A theoretical approach consisted of geometrical-topological analysis, bond valence site energy, kinetic Monte-Carlo (KMC) modeling and density functional theory (DFT) calculations. The theoretical results have shown that oxygen ionic conductivity is possible in all oxymolybdates with DFT migration energy <1.39 eV. We also calculated oxygen ionic conductivity at various temperatures (500–800 °C) using KMC modeling and found values higher than 10–3.5 S/cm at 800 °C, which was in agreement with the experimental measurements. The total conductivity achieved ~10−5 S/cm for Sm2MoO6, ~10−4 S/cm for Gd2MoO6 and ~10−3 S/cm for Dy2MoO6 at 800 °C according to impedance spectroscopy data. The oxygen pressure isotherms in the Ln2MoO6 (Ln = Sm, Gd, Dy) indicated electronic conductivity contribution in the temperature range of 500–800 °C. The experimental electromotive force (EMF) method also showed an increase in the contribution of electronic conductivity with the temperature growth. The activation energy of oxygen ionic diffusion according to EMF data was in the range of 0.78–1.27 eV.