The mechanism of water oxidation performed by a recently discovered manganese pyridinophane catalyst [Mn(Py$_2$N$^t$Bu$_2$)(H2O)$_2$]${^2}$ is studied using density functional theory methods. A complete catalytic cycle is constructed and the catalytically active species is identified to consist of a Mn$^V$-bis(oxo) moiety that is generated from the resting state by a series of proton-coupled electron transfer reactions. Whereas the electronic ground state of this key intermediate is found to be a triplet, the most favorable pathway for O–O bond formation is found on the quintet potential energy surface and involves an intramolecular coupling of two oxyl radicals with opposite spins bound to the Mn-center that adopts an electronic structure most consistent formally with a high-spin Mn$^{III}$ ion. Therefore, the thermally accessible high-spin quintet state that constitutes a typical and innate property of a first-row transition metal center plays a critical role for catalysis. It enables facile electron transfer between the oxo moieties and the Mn-center and promotes O–O bond formation via a radical coupling reaction with a calculated reaction barrier of only 14.7 kcal mol$^{–1}$. This mechanism of O–O coupling is unprecedented and provides a novel possible pathway to coupling two oxygen atoms bound to a single metal site.