Rechargeable magnesium ion batteries have recently received considerable attention as an alternative to Li- or Na- ion batteries. Understanding defects and ion transport is a key step in designing high performance electrode materials for Mg-ion batteries. Here we present a classical potential based atomistic simulation study of defects, dopants and Mg-ion transport in Mg6MnO8. The formation of Mg-Mn anti-site defect cluster is calculated to be the lowest energy process (1.73 eV/defect). The Mg Frenkel is calculated to be the second most favourable intrinsic defect and its formation energy is 2.84 eV/defect. Three-dimensional long range Mg-ion migration path with overall activation energy of 0.82 eV is observed suggesting that the diffusion of Mg-ions in this material is moderate. Substitutional doping of Ga on the Mn site can increase the capacity of this material in the form of Mg interstitials. The most energetically favourable isovalent dopant for Mg is found to be Fe. Interestingly, Si and Ge exhibit exoergic solution enthalpy for doping on the Mn site requiring experimental verification.