The electronic and structural properties of small monomagnesium oxide clusters, MgOx- and MgOx (x = 1-4), have been investigated using a synergistic approach combining photoelectron imaging spectroscopy and first principles electronic structure calculations. The adiabatic detachment energy (ADE) and vertical detachment energy (VDE) of MgOx- clusters along with the photoelectron angular distributions (PADs) are determined experimentally. The measured PADs of the clusters are dependent on both the orbital symmetry and electron kinetic energies. Density-functional theory (DFT) calculations were performed to explore the optimized geometries of neutral and anionic MgOx clusters. The theoretical ADE and VDE values calculated according to the optimized geometries are in good agreement with our experimental measurements. In addition, MgO- and MgO4 clusters are found to have enhanced relative stability in the corresponding anionic and neutral series, based on both theoretical parameters and the experimental cluster distribution.