Manganese oxide supported on activated carbon (MnOx/AC) was prepared by in situ reduction of permanganate with activated carbon. The effects of Mn loading on the morphology, oxidation state, and crystal phase of the supported MnOx were investigated. When the nominal Mn loading was low at 0.44% and 1.1%, the supported MnOx on the surface of activated carbon grew into a porous lichen-like morphology, the oxidation state of Mn was +2.9 to +3.1, and the phase was poorly crystallized β-MnOOH. When the nominal Mn loading was increased to 5.5% and 11%, the lichen-like structure collapsed, the MnOx film mostly consisted of nanospheres, the oxidation state of Mn increased to +3.7 to +3.8, and δ-MnO2 appeared as the main crystal phase. Although the effect of the crystal phase on the activity cannot be excluded, the reason why the 1.1% MnOx/AC catalyst showed the best activity was mainly due to its porous lichen-like structure and relatively high Mn content, while 11% MnOx/AC showed the lowest activity due to its compact MnOx layer even though it had the highest Mn loading. These findings throw light on how to synthesize manganese oxide catalysts with higher activity for ozone decomposition.