Experimental magneto-optical Kerr effect spectra of Mn-implanted Ge samples are interpreted using an ab-initio theoretical model based on density functional theory. The presence of Mn in the Ge matrix is accounted for considering either Mn precipitation as Mn 5 Ge 3 nanoparticles or as perfect dilution. The Mn concentration profile in the sample is also considered in the theoretical model. The computational results are in good qualitative agreement if Mn 5 Ge 3 nanocluster formation is assumed. The effect of partial etching of the implanted region is also studied by comparing our experimental and theoretical MOKE spectra data.