Control over the fabrication of state-of-the-art portable pseudocapacitors with the desired transparency, mechanical flexibility, capacitance, and durability is challenging, but if resolved will have fundamental implications. Here, defect-rich Mn 1−x−y (Ce x La y )O 2−δ ultrathin films with controllable thicknesses (5-627 nm) and transmittance (≈29-100%) are fabricated via an electrochemical chronoamperometric deposition using a aqueous precursor derived from end-of-life nickel-metal hydride batteries. Due to percolation impacts on the optoelectronic properties of ultrathin films, a representative Mn 1−x−y (Ce x La y ) O 2−δ film with 86% transmittance exhibits an outstanding areal capacitance of 3.4 mF cm −2 , mainly attributed to the intercalation/de-intercalation of anionic O 2− through the atomic tunnels of the stratified Mn 1−x−y (Ce x La y )O 2−δ crystallites. Furthermore, the Mn 1−x−y (Ce x La y )O 2−δ thin-film device exhibits excellent capacitance retention of ≈90% after 16 000 cycles. Such stability is associated with intervalence charge transfer occurring among interstitial Ce/ La cations and Mn oxidation states within the Mn 1−x−y (Ce x La y )O 2−δ structure. The energy and power densities of the transparent flexible Mn 1−x−y (Ce x La y ) O 2−δ full-cell pseudocapacitor device, is measured to be 0.088 μWh cm −2 and 843 μW cm −2 , respectively. These values show insignificant changes under vigorous twisting and bending to 45-180° confirming these value-added materials are intriguing alternatives for size-sensitive energy storage devices.