In this study, the Mg doping effects on the nanostructural dependence of photoluminescence and the photo-sensing properties in Zn1-xMgxO (x = 0.01, 0.03, and 0.05) nanorod arrays were systematically examined. The vertical-aligned nanorod arrays were separately grown on ZnO-seeded n-Si substrates via a low-temperature hydrothermal process. X-ray diffraction analysis revealed that all Zn1-xMgxO nanorod arrays exhibited the same preferentially c-oriented wurtzite structure. Images of all the Zn1-xMgxO nanorod arrays provided by field emission scanning electron microscope showed that both the length and diameter of the Zn1-xMgxO nanorod arrays increased with increasing Mg doping concentration. Also, an enhanced blue-shifted ultraviolet (UV) emission and a reduced visible emission were observed from the photoluminescence spectra of the Zn1-xMgxO nanorod arrays. The former indicated the decrease of defects and the latter indicated the enhancement of the crystallinity and the increase in optical bandgap. Current-voltage curves were separately measured in the dark and under UV illumination. Finally, the mechanism of the Mg doping effects on the corresponding defects, bandgap, and UV photosensing characteristics of the Zn1-xMgxO nanorod arrays was investigated in detail.