Nanocomposite Cu–Mn–O nano-particle (CuMnNP) and nano-sheet (CuMnNS) catalyst were successfully prepared using a one-step hydrothermal method in the absence of any templating reagent. Materials were characterised applying various structural techniques. SEM images showed that composite Cu–Mn oxide sheets were tailor-made synthesised by a one-pot urea-abetted protocol. Conversely, upon replacing carbamate by Na2CO3, oxidised metal Cu–Mn particles could be obtained. The formation of bulk mixed Cu–Mn phases resulted in an enhanced crystal lattice oxygen reactivity in CuMnNS. XPS, XRD and TPR measurements confirmed the presence of the Cu+ and Cu2+ species in nano-catalysts, and CuMnNS nanomaterials possessed more surface defects, thus causing a higher O2 adsorption/storage capacity. CuMnNS presented a superior catalytic activity as opposed to CuMnNP in the preferential oxidation (PROX) pathway of CO. With both CO2 and H2O in feed, a decrease in CO turnover was observed, due to a competitive interface binding of CO, CO2 and H2O. Compared to CuMnNP, CuMnNS also demonstrated a high time-on-stream conversion of methanol for the reforming for all operating conditions.