The low voltage-operational thin-film type of the high-quality nanoporous ZnMnO (Mn: similar to 18 at.%) green phosphor was fabricated by grain boundary engineering via the thermal nucleation of the well-crystallized co-lumnar ZnMnO thin film. Vertically-aligned nanopores were observed to be formed throughout the whole area of the ZnMnO phosphor film; and the impact excitation of Mn2+ centers and the defect-mediated optical transition in nanoporous ZnMnO significantly enhanced green light emission in the nanoporous cavity. The results suggest nanoporous ZnMnO to hold promise for the next-generation color-field panels as a new type of the thin-film phosphor. The material characteristics, including the structural, electrical, and optical properties, are meticulously examined, and the formation mechanism of nanoporous ZnMnO and the origin of its low-voltage strong-green-cathodoluminescence are described.