In this work, we propose a systematic and delicate size- and shape-controlled synthesis of CuO-MnO2 composite nanostructures from time-dependent redox transformation reactions between Cu2O and KMnO4. The parental size and shape of Cu2O nanostructures are retained, even after the redox transformation, but the morphology becomes porous in nature. After prolonged reaction times (>24 h), the product shapes are ruptured, and as a result, tiny spherical porous nanocomposites of ∼100 nm in size are obtained. This method is highly advantageous due to its low cost, its easy operation, and a surfactant or stabilizing agent-free approach with high reproducibility, and it provides a facile but new way to fabricate porous CuO-MnO2 nanocomposites of varied shape and size. The composite nanomaterials act as efficient recyclable catalysts for nitroarene reduction in water at room temperature. The time-dependent reduction kinetics can be easily monitored by using UV-vis spectrophotometer. The catalytic system is found to be very useful toward the reduction of nitro compounds, regardless of the type and position of the substituent(s). Furthermore, it is revealed that CuO-MnO2 composite nanomaterials exhibit facet-dependent catalytic activity toward nitroarene reduction, where the (111) facet of the composite stands to be more active than that of the (100) facet. The results are also corroborated from the BET surface area measurements. It is worthwhile to mention that porous tiny spheres (product of 48 h reaction) exhibit the highest catalytic activity due to pronounced surface area and smaller size.