The effect of Mn doping in the electronic structure of cubic KTaO3 has been studied by first-principles calculations, based on density functional theory. In our study, we analyze different Mn-dopant concentrations, namely 6.25 %, 12.5 %, and 25 %, with Mn replacing Ta, K, or both sites, and probe changes in electrical, magnetic, and optical properties. Our results show that for Mn substituting Ta or K atoms, the structures acquire a metallic character and, when Mn substitutes both Ta and K atoms, the structures conserve a semiconductor character. On the other hand, all doped structures acquire a ferromagnetic character and magnetic moment resides almost entirely in Mn atom, being larger when Mn substitutes K atoms. Analysis of the calculated optical properties, allows us to monitor changes in the absorption coefficient. We show that structures for Mn replacing both Ta and K sites display an increased absorption of electromagnetic radiation in the visible region. Analyzing the formal oxidation states, following a Bader's analysis of component atoms, allows us to explain changes in electrical, magnetic, and optical properties.