The effect of surface polarity on the growth of Mg-doped GaN thin films on c-plane sapphire substrates by molecular-beam epitaxy has been investigated. The doping behavior of Mg and resulting conductivity of the doped layers were found to strongly depend on the surface polarity of the growing GaN planes. The samples grown on the Ga-polar face (A face) exhibited a p-type conductivity with a free-hole concentration up to 5×1017 cm−3, while the samples grown on the N-polar face (B face) were highly resistive or semi-insulating. The incorporation of residual impurities (O, Si, and C) in the two different polar surfaces was studied by secondary ion mass spectrometry analysis and its effect on the Mg doping was discussed. Our results suggest that the A face (Ga face) is the favored surface polarity for achieving p-type conductivity during the growth of Mg-doped GaN.