Abstract: This work systematically investigates the role of catalyst Oxygen Vacancies (O_V) in plasma-catalyzed ammonia synthesis over MnO_x/MgAl Layered Double Hydroxide (LDH). Experiments show that the MnO_x/LDH-500 catalyst (where -500 indicates calcination at 500 °C) with the highest surface oxygen vacancy concentration achieves an ammonia concentration of 12056 parts per million (ppm), which is 1.54 times that obtained with the bare LDH support possessing the lowest vacancy content, 1.27 times that of MnO_x/LDH-300 and 1.33 times that of MnO_x/LDH-700. Meanwhile, the energy yield increased significantly with increase in oxygen vacancies. The MnO_x/LDH-500 catalyst achieved an energy yield of 1.10 g·kWh⁻¹, which is 1.54 times that of the bare LDH support. In situ optical diagnostics reveal that the key intermediate, excited N₂, is first generated in the gas phase and then adsorbed onto the catalyst surface to form NH_x species for ammonia formation. The benefit of oxygen vacancies lies in providing surface sites for excited N₂ adsorption. These findings offer scientific guidance and a practical strategy for designing efficient catalysts compatible with plasma processes.