Active and stable electrocatalysts for the oxygen evolution reaction are required to produce hydrogen from water using renewable electricity. Here we report that incorporating Mn into the spinel lattice of Co3O4 can extend the catalyst lifetime in acid by two orders of magnitude while maintaining the activity. The activation barrier of the obtained spinel Co2MnO4 is comparable to that of state-of-the-art iridium oxides, most probably due to the ideal binding energies of the oxygen evolution reaction intermediates, as shown using density functional theory calculations. The calculations also show that the thermodynamic landscape of Co2MnO4 suppresses dissolution, which results in a lifetime of over 2 months (1,500 hours) at 200 mA cm−2geo at pH 1. As the lifetimes of other 3d metal oxygen evolution catalysts are in the order of days and weeks, despite current densities being lower by an order of magnitude, our results are an important step towards the realization of noble-metal-free water electrolysers.