In this work, we have investigated the electrochemical ion insertion redox behavior of A x MnO 2 compounds (A = H + , Li + , Na + , K + , Mg 2+ , Ca 2+ , Zn 2+ & Al 3+ ) in aqueous and non-aqueous environments using ab-initio density functional theory. We show that Hubbard-U corrections are necessary for computing accurate redox energetics, and that these corrections are more important than choice of exchange-correlation functional (PBE vs. SCAN). Using PBE+U, we find that nonprotonic cation insertion into MnO 2 strongly depends on how effectively ions are coordinated by oxygen atoms inside the tunnel and layered structures. A competing driving force to form O-H bonds exists whenever H + is present and is usually favored over non-protonic cation insertion. This finding explains experimental reports of H + and H + /Zn 2+ co-insertion mechanisms in aqueous Znion batteries. 𝛼-K 0.125 MnO 2 , 𝛼-Na 0.125 MnO 2 , and λ-Li 0.5 MnO 2 are the only phases with inserted nonprotonic cations that are thermodynamically stable within water's voltage stability window, while other aqueous ion insertion in MnO 2 relies on metastability. We identify Al 3+ insertion into the 𝛿, R, and λ polymorphs as promising candidates to achieve the full 2 electron redox of MnO 2 , however, multivalent ion battery electrolytes that can prevent the precipitation of insoluble (hydr)oxides and facilitate ion desolvation will likely have to be developed. We also show that coinsertion with water is favored for small ions in the 𝛼 polymorph due to confinement effects, while solvation energies and kinetic effects dictate water insertion in 𝛿-MnO 2 . In both cases, capacity decreases due to water molecules occupying sites that could otherwise host an inserted cation. The calculations performed in this work rationalize experimental observations of MnO x electrochemical behavior and serve as a guide for the rational design of Mn oxide ion-insertion devices.