Epitaxial thin films of SrTiO₃(100) doped with 6% and 12% Ni are studied with resonant angle-resolved photoelectron spectroscopy at the Ti and Ni L_2,3-edges. We find that the Ni doping shifts the valence band of n-doped pristine SrTiO₃ toward the Fermi level (in the direction of p-doping) and reducing the bandgap. In the Ti t_2g-derived mobile electron system (MES), the Ni doping depopulates the out-of-plane d_xz/yz-derived bands, transforming the MES to two-dimensional and progressively reduces the electron density embedded in the in-plane d_xy-derived bands as reflected in their Fermi momentum. Furthermore, the Ti and Ni L_2,3-edge resonant photoemission is used to identify the Ni 3d impurity state in the vicinity of the valence-band maximum and decipher the full spectrum of the in-gap states originating from the Ni atoms, Ti atoms, and from their hybridized orbitals. Our experimental information about the dependence of the valence bands, MES, and in-gap states in Ni-doped SrTiO₃ may help the development of this material toward its device applications associated with the reduced optical bandgap.