AbstractAt crystalline interfaces where a valence-mismatch exists, electronic, and structural interactions may occur to relieve the polar mismatch, leading to the stabilization of non-bulk-like phases. We show that spontaneous reconstructions at polar La_0.7Sr_0.3MnO₃ interfaces are correlated with suppressed ferromagnetism for film thicknesses on the order of a unit cell. We investigate the structural and magnetic properties of valence-matched La_0.7Sr_0.3CrO₃/La_0.7Sr_0.3MnO₃ interfaces using a combination of high-resolution electron microscopy, first principles theory, synchrotron X-ray scattering and magnetic spectroscopy and temperature-dependent magnetometry. A combination of an antiferromagnetic coupling between the La_0.7Sr_0.3CrO₃ and La_0.7Sr_0.3MnO₃ layers and a suppression of interfacial polar distortions are found to result in robust long-range ferromagnetic ordering for ultrathin La_0.7Sr_0.3MnO₃. These results underscore the critical importance of interfacial structural and magnetic interactions in the design of devices based on two-dimensional oxide magnetic systems.