Significance Transition metal oxide interfaces have shown extraordinary promise in the quest to design materials with custom electronic, magnetic, and optical properties. In rare cases, interfaces exhibit electronic and magnetic properties that are radically different from those of the components. An example is the emergent two-dimensional electron gas between two insulators. In this work, we show that the interface of a nonmagnetic oxide substrate and a ferromagnetic metallic thin film possesses a local antiferromagnetic coupling which controls the reversal of the entire film’s ferromagnetic ordering. Electron microscopy and quantum calculations elucidate the atomic-scale origin of the observed phenomena, and demonstrate that local nonstoichiometry and structure are the key factors in interfacial phenomena.