jats:pWe report the fabrication and electrochemical performance of a flexible thin film supercapacitor with a novel nanostructured composite electrode. The electrode was prepared by jats:italicin situ</jats:italic> coprecipitation of two‐dimensional (2D) MnOjats:sub2</jats:sub> nanosheets at room temperature in the presence of carbon nanofibers (CNFs). The highest specific capacitance of 142 F/g was achieved for CNFs‐MnOjats:sub2</jats:sub> electrodes in sandwiched assembly with PVA‐Hjats:sub4</jats:sub>SiWjats:sub12</jats:sub>Ojats:sub40</jats:sub>· jats:italicn</jats:italic>Hjats:sub2</jats:sub>O polyelectrolyte separator.</jats:p>