Pure and Mg-doped manganese oxide thin films were synthesized on heated glass substrates using the spray pyrolysis technique. The surface chemical composition was investigated by the use of X-ray photoelectron spectroscopy (XPS). Structural and morphological properties were studied by using X-ray diffraction (XRD), scanning electron microscope (SEM) and atomic force microscopy (AFM). Optical properties were characterized by UV-visible spectroscopy. XPS spectra showed typical Mn (2p3/2), (2p1/2) and O (1s) peaks of Mn3O4 with a slight shift attributed to the formation of different chemical states of manganese. XRD analysis revealed the tetragonal phase of Mn3O4 with a preferred (211) growth orientation that improved with Mg-doping; likewise, grain size is observed to increase with the Mg doping. SEM images of Mn3O4 films showed rough surfaces composed of uniformly distributed nanograins whose size decreases with the Mg-doping. The manganese oxide films surface observed in AFM show a textured, rough and porous surface. The combination of transmittance and absorption data in the UV-visible range allowed determining the energy values of the Eg band gap (1.5–2.5 eV). The decrease of the band gap with the Mg-doping increase is attributed to the influence of the greater size of the Mg2+ ion in the manganese oxide lattice.