We demonstrate the application of the Sagnac interferometer to magneto-optic measurements at an oblique angle of incidence. With an appropriate choice of polarization states for the two counter-propagating beams, a wide variety of magneto-optic effects may be measured, in the absence of an external perturbing field, with a sensitivity of a few hundred nanoradians. Using simple symmetry analysis of the conventional Kerr rotation measurement geometry, we may distinguish contributions to the non-reciprocal phase shift due to the polar, longitudinal and transverse Kerr effects, and consequently completely determine the magnetization vector direction, averaged over the probed region. Magneto-optic hysteresis loops were taken on a permalloy film to demonstrate the effectiveness of the new technique. We discuss the relevance of the Sagnac interferometer to magnetic microscopy and to the study of magnetic anisotropies in thin films. !