A mixed ionic-electronic conductor (MIEC) plays a crucial role in electrochemical technologies relevant to energy conversion and storage. The existing composite materials often suffer from poor mixed conducting performance due to the distinct phase boundaries and random distributions of the transport channels. Herein, we propose the concept for the fabrication of a single-phase MIEC using two-dimensional (2D) building blocks-phosphorylated graphene nanosheets, which are assembled into three-dimensional (3D) interconnected networks with long-range ordered nanochannels. Attributed to the sufficient proton carriers (phosphate groups) confined in electron-conductive nanochannels and the integration of conductive pathways, simultaneously enhanced proton conductivity (0.13 S cm-1) and electron conductivity (0.265 S cm-1) are achieved at 98% RH and 35 °C, surpassing the current performance of all graphene-based MIECs. This approach may pave the way for designing MIECs with high mixed conduction by utilizing the unique properties of 2D materials, beyond the limitations of pure proton-conducting/electron-conducting materials.