Amorphous metal oxides have numerous applications spanning from electronics to catalysis. The ability to rationally design the electronic structure of such materials would thus have significant impact. Previous work has shown that the amorphous structure allows for uniformly incorporating a large number of different metal cations into ultrasmooth thin films. Here we investigate how the atomic structure, electronic structure, and optical absorption properties of amorphous aluminum oxide (a-Al2O3) thin films are modulated when metal cations of V, Cr, Mn, Fe, Co, Ni, Cu, Zn, or Ga are added to form MyAl1–yOx where y varies from 0.1 to 0.7. We use X-ray absorption spectroscopy (XAS) to analyze oxidation state and local bonding around the incorporated cations (e.g., coordination number), valence-band X-ray photoelectron spectroscopy (VB-XPS) to assess the impact of the cations on the valence band electronic structure, and optical absorption spectroscopy to assess the presence and energies of new unoccupied electronic states. Cations with partially filled d-orbitals (e.g., Fe3+, Cr3+, Co2+, Ni2+, Cu2+, etc.) add filled 3d electronic states at the valence band edge of a-Al2O3 as well as unfilled 3d electronic states within the a-Al2O3 band gap. Cations with the d10 electronic configuration, such as Zn2+ and Ga3+, introduce filled 3d electronic states at energies well below the valence band edge in addition to new unfilled electronic states in the a-Al2O3 band gap. Cations with unfilled d-orbitals (e.g., those with a d0 configuration such as V5+ and Cr6+) do not modify the valence band of a-Al2O3, but affect the optical properties by inserting unfilled 3d electronic states within the band gap. These results provide guidance for the rational design of the electronic structure of amorphous mixed-metal oxides.