A novel method for the electrodeposition of highly active water oxidation catalysts is described. The manganese oxide (MnOx) films are electrodeposited on fluorine tin oxide (FTO) glass substrate at high temperature (120 °C) from an ionic liquid electrolyte (ethylammonium nitrate). A range of analytical techniques, including X-ray absorption spectroscopy (XAS), X-ray diffraction (XRD), and energy-dispersive X-ray analyzer (EDX), indicate that the valence state of manganese in the deposited films can be controlled by changing the electrolyte composition. Along with the different phase compositions, a number of different morphologies including nanowires, nanoparticles, nanofibers as well as highly open and dense structures are obtained by varying the acidity of the electrolyte. The effect of morphology and chemical composition on the catalytic activity towards water oxidation is investigated. The film composed of Mn3O4 shows low catalytic activities, while the films composed of birnessite-like manganese oxide phase and Mn2O3 exhibit high catalytic activities for water oxidation. The catalytic activities are also affected by the surface morphology, i.e., a higher surface area and more open structure shows a higher catalytic activity. High rates of oxygen production are observed from MnOx films prepared in a neutral electrolyte.