MnO2 is a versatile, cost-effective transition metal oxide that has attracted interest in multiple domains, including as an active cathode material or catalyst for electrochemical energy storage in batteries. Hydrothermal methods are among the most efficient approaches for MnO2 synthesis. These approaches enable facile, versatile production of MnO2 in any of its crystalline phases (α, β, δ, etc.), with the dominant product being determined by reaction conditions such as precursor concentration and temperature. These benefits unfortunately come with impractically low product yields (~9%) and long reaction times. Here, we report that low-molecular weight, water-soluble polymers function as effective nucleating agents for the hydrothermal synthesis of MnO2. For fundamental reasons, these polymers are also reported to promote large (10- to 40-fold) increases in reaction rate and yield. We evaluate the physical and crystallographic features of the synthesized MnO2 and find that depending upon the reaction conditions, the polymer-assisted synthesis yields dominantly δ-MnO2 or β-MnO2. The as-prepared δ-MnO2 materials with extra interlayer water are studied as electrodes for aqueous Zn-ion battery applications are shown to support long-duration storage at both moderate and high rates.