Preparation of single-layer manganese oxide nanosheets (monosheets) comprised of edge-shared MnO6 octahedra has relied on multistep processing involving a high-temperature solid-state synthesis of bulk templates, and ion-exchange and exfoliation reactions in solutions, requiring high cost and long processing time. Here we demonstrate the first single-step approach to directly access the MnO2 monosheets, by the chemical oxidation of Mn2+ ions in the presence of tetramethylammonium cations in an aqueous solution. Of importance is that this template-free reaction readily proceeds within a day at room temperature. The ability of the MnO2 monosheets to self-assemble allows aggregation, to form layered structures with potassium cations and cationic tetrathiafulvalene analogues as intercalants. Furthermore, Langmuir-Blodgett (LB) films composed of the MnO2 monosheets were successfully fabricated by the LB deposition method, in which about one layer of the monosheets was deposited for each process.