Polytypism, or stacking disorder, in crystals is an important structural aspect that can impact materials properties and hinder our understanding of the materials. One example of a polytypic system is todorokite-MnO2, which has a unique structure among the transition-metal oxides, with large ionic conductive channels formed by the metal oxide framework that can be utilized for potential functionalization, from molecular/ion sieving to charge storage. In contrast to the perceived 3 × 3 tunneled structure, we reveal a coexistence of a diverse array of tunnel sizes in well-crystallized, chemically homogeneous one-dimensional todorokite-MnO2. We explain the formation and persistence of this distribution of tunnel sizes thermochemically, demonstrating the stabilization of a range of coherent large-tunnel environments by the intercalation of partially solvated Mg2+ cations. Based on structural behavior of the system, compared to the common well-ordered alkali-stabilized polymorphs of MnO2, we suggest generalizable principles determining the selectivity of structure selection by dopant incorporation.