Accurate ab initio calculations are performed to investigate the stable isomers of [MgO3]+ and Q1 its lowest electronic states at both molecular and asymptotic regions. The calculations are done using large basis sets and configuration interaction methods including the complete active space self-consistent field, the internally contracted multi-reference configuration interaction, the standard coupled cluster (RCCSD(T)) approaches and the newly implemented explicitly correlated coupled cluster method (RCCSD(T)-F12). The presence of three stable forms is predicted: a cyclic global minimum c-MgO3 +, which is followed by a quasi-linear isomer, l2-MgO3 +. A third isomer of Cs symmetry (l1-MgO3 +) is also found. Moreover, we computed the one-dimensional cuts of the sixdimensional potential energy surfaces of the lowest doublet and quartet electronic states of [MgO3]+ along the RMgO and ROO stretching coordinates covering both the molecular and the asymptotic regions. These curves are used later for discussing the metastability of this cation and to propose plausible mechanisms for the Mg+ + O3 atmospherically important ion-molecule reaction and related reactive channels.