The phase relationships between the vector components of a time-varying field define its polarization, which can provide insight into the structure of a magnetic field source and its environment. The goal of this work is to measure magnetic field elliptical polarization parameters, and demonstrate their utility through direct observation of field source rotations and environmental coupling. An ensemble of nitrogen-vacancy (NV) centers in diamond is used to perform phase-sensitive ac vector magnetometry, enabling direct measurement of field polarization ellipses using complex spin-resonance solutions to the NV Hamiltonian. Measurements of wire-coil sources demonstrated retrieval of linear and elliptical magnetic polarizations, allowing direct observation of source rotation and mutual coupling between coils. These results establish magnetic polarimetry as a practical extension of ac vector magnetometry, and promise applications in vector ac susceptometry for anisotropic materials characterization, and the analysis of reactive electrical systems.