Covalency is a central concept in inorganic chemistry and has been invoked to explain redox potentials, magnetic exchange, and chemical reactivity of inorganic complexes. Experimentally investigating metal-ligand covalency is often, however, a challenging task. Commonly used techniques include EPR-based methods as well as ligand K-edge and metal L-edge x-ray absorption, though these methods all impose often severe limitations on the kinds of samples that may be investigated. Using experimental data obtained from Beamline 6-2 at SSRL, we have added another technique to this toolbox by establishing Kβ mainline x-ray emission spectroscopy (XES) as a quantitative probe of metal-ligand covalency. Kβ mainline spectra result from metal-centered 3p to 1s transitions and show a strong dependence on the number of unpaired 3d electrons on the metal center (via the 3p-3d exchange integrals). This same sensitivity to the 3p-3d exchange also makes Kβ mainlines sensitive to the metal-ligand covalency, as covalent delocalization of the 3d electrons onto the ligands effectively reduces the magnitude of this exchange. A series of experiments performed at Beamline 6-2 on high spin ferric halides (fluoride, chloride, bromide) dramatically demonstrated this sensitivity to covalency, as the mainline spectra shifted by almost 3 eV across the series, a change greater than would be expected for a change in metal oxidation. These experimental results were supported by restricted active space configuration interaction (RAS-CI) calculations that reproduced the experimental data and allowed for a quantitative correlation of the data to various computational measures of metal-ligand covalency. Experimentally, Kβ mainline XES presents numerous benefits over existing techniques for determining covalency, including selectivity for any element of interest and an applicability to a wide range of sample environments (e.g. high pressure cells, dilute protein solutions). With this broad applicability, Kβ mainline XES holds promise to become a widely used probe of metal ligand covalency.