We present a detailed study of the magnetic, spectroscopic and structural properties of ultrathin LaMnO₃ films deposited on SrTiO₃(001) substrates by oxide molecular beam epitaxy. We find that the as-grown LaMnO₃ films are slightly reduced and present a significant magnetic moment, while annealing to 600 °C fully oxidizes and enhances its magnetic properties. From synchrotron x-ray photoemission spectromicroscopy, we find the presence of Sr, Ca and Si in the LaMnO₃ film that diffuse from the SrTiO₃ substrate; these impurities act as hole dopants, and can explain the presence of ferromagnetism in LaMnO₃. This work highlights the importance of cation mobility at the elevated growth temperature in modifying the magnetic properties of ultrathin LaMnO₃ films.