Interfaces between transition metal oxides are known to exhibit emerging electronic and magnetic properties. Here we report intriguing magnetic phenomena for La_2/3Sr_1/3MnO₃ films on an SrTiO₃ (001) substrate (LSMO/STO), where the interface governs the macroscopic properties of the entire monolithic thin film. The interface is characterized on the atomic level utilizing scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS), and density functional theory (DFT) is employed to elucidate the physics. STEM-EELS reveals mixed interfacial stoichiometry, subtle lattice distortions, and oxidation-state changes. Magnetic measurements combined with DFT calculations demonstrate that a unique form of antiferromagnetic exchange coupling appears at the interface, generating a novel exchange spring-type interaction that results in a remarkable spontaneous magnetic reversal of the entire ferromagnetic film, and an inverted magnetic hysteresis, persisting above room temperature. Formal oxidation states derived from electron spectroscopy data expose the fact that interfacial oxidation states are not consistent with nominal charge counting. The present work demonstrates the necessity of atomically resolved electron microscopy and spectroscopy for interface studies. Theory demonstrates that interfacial nonstoichiometry is an essential ingredient, responsible for the observed physical properties. The DFT-calculated electrostatic potential is flat in both the LSMO and STO sides (no internal electric field) for both Sr-rich and stoichiometric interfaces, while the DFT-calculated charge density reveals no charge transfer/accumulation at the interface, indicating that oxidation-state changes do not necessarily reflect charge transfer and that the concept of polar mismatch is not applicable in metal−insulator polar−nonpolar interfaces. 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.