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Initiating anionic redox chemistry in layered sodium oxide cathodes is a prevalent method to break the capacity limit set by traditional transition metal redox. However, realizing the "win‐win" scenario of high capacity and high cycling stability is still challenging due to the high‐voltage structural distortion and irreversible oxygen loss. Herein, a Mn activation mechanism is unveiled in a novel P2‐Na0.80Li0.08Ni0.22Mn0.67O2 cathode. By elevating the charge cut‐off voltage to 4.3 V, anionic redox is successfully triggered and partial oxygen loss enables the reduction of Mn upon discharge, thus activating more Mn3+/Mn4+ redox reactions in the following cycles and maintaining the total capacity almost unchanged. In situ X‐ray diffraction reveals a complete solid‐solution reaction with an ultralow volume change of 1.04% upon cycling. Consequently, the P2‐Na0.80Li0.08Ni0.22Mn0.67O2 cathode simultaneously accomplishes a high discharge capacity (134.8 mAh g−1 at 0.1 C) and an unexpectedly long cycling life (capacity retention of 91.5% and 85.2% after 500 and 1000 cycles at 10 C, respectively). Via systematic ex situ characterizations and theoretical computations, the charge compensation mechanism upon Na+ insertion/extraction is elucidated. This work broadens the horizons of current oxygen redox chemistry and provides a new path to design high‐performance layered oxide cathode materials for sodium‐ion batteries. [ABSTRACT FROM AUTHOR]