We report a study of the structure and bonding of a transition-metal-doped boron cluster, MnB6-, using high-resolution photoelectron imaging and quantum chemical calculations. Vibrationally resolved photoelectron spectra indicate a significant geometry change between the anionic and neutral ground states of MnB6. The electron affinity of MnB6 is measured to be 2.4591(5) eV, and vibrational frequencies for five of its vibrational modes were determined. The experimental data are combined with theoretical calculations to determine the structure and bonding of MnB6-, which is found to be planar with a B-centered hexagonal structure (C-2v symmetry) and a quintet spin state ((5)A(2)). Nuclear-independent chemical shift calculations indicate that MnB6- is aromatic. Molecular orbital analyses reveal that MnB6- contains three pi orbitals, one of which is singly occupied. Hence, MnB6- can be considered as an open-shell metallaboron analog of 3d metallabenzenes.