In this study, we investigated the influence of polarity on Mg incorporation in ZnO films grown using pulsed laser deposition. The photoluminescence (PL) measurements of (Zn,Mg)O films grown on the c(+) and c(−) faces of ZnO revealed that the peak position of the near-band-edge emission for the films on the c(−)-face was at a shorter wavelength than that for the films on the c(+)-face. This result indicates that the Mg content in the films on the c(−)-face is higher than that for the films on the c(+)-face regardless of the same growth conditions used, except for the polarity of the substrates. This difference in the Mg content in the c(+)- and c(−)-polar films can result from the difference in the value of the sticking coefficients for Zn atoms on the c(+)- and c(−)-faces. We also demonstrated that polarity determination can be possible by (Zn,Mg)O film deposition followed by PL mapping measurement.