A mononuclear non-heme manganese(IV)-oxo complex has been synthesized and characterized using various spectroscopic methods. The Mn(IV)-oxo complex shows high reactivity in oxidation reactions, such as C-H bond activation, oxidations of olefins, alcohols, sulfides, and aromatic compounds, and N-deallcylation. In C-H bond activation, the Mn(IV) oxo complex can activate C-H bonds as strong as those in cyclohexane. It is proposed that C H bond activation by the non-heme Mn(IV)-oxo complex does not occur via an oxygen-rebound mechanism. The electrophilic character of the non-heme Mn(IV) oxo complex is demonstrated by a large negative p value of -4.4 in the oxidation of para-substituted thioanisoles.