Transition metals such as Mn generally have large local moments in covalent semiconductors due to their partially filled d shells. However, Mn magnetization in group-IV semiconductors is more complicated than often recognized. Here we report a striking crossover from a quenched Mn moment (<0.1 μB) in amorphous Si (a-Si) to a large distinct local Mn moment (≥3 μB) in amorphous Ge (a-Ge) over a wide range of Mn concentrations (0.005-0.20). Corresponding differences are observed in d -shell electronic structure and the sign of the Hall effect. Density-functional-theory calculations show distinct local structures, consistent with different atomic density measured for a-Si and a-Ge, respectively, and the Mn coordination number Nc is found to be the key factor. Despite the amorphous structure, Mn in a-Si is in a relatively well-defined high coordination interstitial type site with broadened d bands, low moment, and electron (n -type) carriers, while Mn in a-Ge is in a low coordination substitutional type site with large local moment and holes (p -type) carriers. Moreover, the correlation between N c and the magnitude of the local moment is essentially independent of the matrix; the local Mn moments approach zero when Nc >7 for both a-Si and a-Ge.