The Green function completely encapsulates a system's linear response to external sources, and plays a central role in optics, electromagnetism, and acoustics. In electromagnetism, a broad range of phenomena are connected to the Green function, including the local density of optical states, superradiance, and the cooperative Lamb shift. Therefore, knowing the Green function is important for progress in fields as diverse as cavity quantum electrodynamics, plasmonics, metamaterials, and photovoltaics. However, experimentally characterizing the full complex Green function is challenging, as it requires amplitude and phase sensitive measurements with deep-subwavelength spatial resolution. Here, we report a method to characterize the full complex Green function with a resolution of λ=100 by measuring the mutual impedance between two dipoles at microwave frequencies. We apply it to a resonant planar cavity, with both parallel and nonparallel sides, and also explore the effects of modal resonances in a dielectric cube on dipole-dipole interactions. The ability to characterize the Green function with high spatial resolution provides a unique way to investigate cooperative effects in complex photonic systems.