The crystalline sub-nanometer-cage structure of mayenite (12CaO•7Al2O3) is a playground for various anions, including oxygen and its radicals, hydrogen, and anionic electrons (a characteristic of electrides). These electrons are clathrated inside the nanocages like anions but with a low work function (~2.4 eV), bringing mayenite electride broad applications in transparent conductors, catalysis and electron emitters. The real electron donor has attracted much study since mayenite first exhibited persistent conductivity through hydrogenation and post-UV radiation, but still remains elusive. In this work, we systematically study the reaction paths of hydrogen into mayenite by first-principles simulations. Various hydrogen states, at different positions and valences, are investigated. According to thermodynamics calculations, we suggest a new mechanism of mayenite electride formation based on the competition between protons and hydrides. This research uncovers the role of multi-state hydrogen in electron localization and delocalization, which inspires a further understanding into electrides.