We present experimental final-state distributions for Mg atoms formed in Mg++D− mutual neutralization reactions at center-of-mass collision energies of 59±12  meV by using the merged-beams method. Comparisons with available full-quantum results reveal large discrepancies and a previously underestimated total rate coefficient by up to a factor of 2 in the 0–1 eV (<104  K) regime. Asymptotic model calculations are shown to describe the process much better and we recommend applying this method to more complex iron group systems; data that is of urgent need in stellar spectral modeling.