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 Co 3 O 4 can extend the catalyst lifetime in acid by two orders of magnitude while maintaining the activity. The activation barrier of the obtained spinel Co 2 MnO 4 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 Co 2 MnO 4 suppresses dissolution, which results in a lifetime of over 2 months (1,500 hours) at 200 mA cm −2 geo at pH 1. As the lifetimes of other 3 d 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.