Nanomaterials having enzyme-like activity (nanozymes) make them suitable candidates for various biomedical applications. In this study, we demonstrate the morphology-dependent enzyme mimetic activity of Mn O nanoparticles. It is found that Mn O nanoparticles mimic the functions of all three cellular antioxidant enzymes: superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx). Interestingly, the nanozyme activity of Mn O depends on various factors including size, morphology, surface area, and the redox properties of the metal ions. The Mn O nanoflowers exhibited remarkably high activity in all three enzyme systems and the order of multienzyme activity of different morphologies was: flowers ≫ flakes > hexagonal plates≈polyhedrons≈cubes. Interestingly, all five nanoforms are taken up by the mammalian cells and were found to be biocompatible, with very low cytotoxicity. The activity of the most active nanoflowers was studied in primary human umbilical vein endothelial cells (HUVEC) and human pulmonary microvascular endothelial cells (hPMEC) and it was found that Mn O does not reduce the level of nitric oxide (NO). This is in contrast to the effect of some of the Mn-porphyrin-based SOD mimetics, which are known to scavenge NO in endothelial cells.