We have studied the electronic structure of MnO by photoemission spectroscopy. Mn 3d--derived emission is found to be confined within \ensuremath{\sim}10 eV of the top of the valence band, and we find no evidence for an intrinsic satellite at higher binding energies. The photoemission intensity of the whole valence band is enhanced for photon energies in the Mn 3p\ensuremath{\rightarrow}3d core absorption region. These observations suggest that ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L photoemission final states (L denotes a ligand hole) are close in energy and are strongly hybridized with each other. The spectra are analyzed in terms of configuration-interaction theory using a ${\mathrm{MnO}}_{6}$ cluster model. We thus find that the ligand-to-Mn d charge-transfer energy \ensuremath{\Delta} is comparable to the intra-atomic Coulomb energy U\ensuremath{\simeq}7.5 eV. Namely, MnO is indeed close to the boundary between the Mott-Hubbard (U) and the charge-transfer (Ug\ensuremath{\Delta}) regimes in the Zaanen-Sawatzky-Allen phase diagram, and the ${\mathit{d}}^{4}$ and ${\mathit{d}}^{5}$L states are nearly degenerate. The large \ensuremath{\Delta} value leads to a highly ionic character in the Mn-O bonding.