The present work reports an efficient, sustainable, and cost-effective chemical route for gram-scale synthesis of heterobimetallic Co–Mn oxide (Co2Mn3O8) from the redox reaction between in situ-generated zero-valent cobalt nanomaterial and KMnO4 in water. The magnetically recoverable Co2Mn3O8 nanomaterial showed promising catalytic activity and good selectivity (>99%) for the oxidation of alcohols to aldehydes/ketones both in the presence of air and hydrogen peroxide for a variety of alcohols including benzyl, aliphatic, cinamyl, pyridine, and thiophene moieties. The inexpensive Co2Mn3O8 furnishes excellent catalytic activity and chemoselectivity (>99%) for the reduction of a wide range of aromatic and heteroaromatic nitro substrates to corresponding amines and various dyes under relatively milder reaction conditions with high turnover frequency (TOF). The high catalytic performance and durability in the Co2Mn3O8 nanomaterial for both reactions in comparison to their monometallic oxide (MnO2 and Co3O4) is further attributed to the synergistic effects between each component. Easy synthesis, large-scale application, excellent selectivity, effortless separation of the catalyst using an external magnet, and efficient recycling make the catalyst and the protocol economical and sustainable.