The growing demand for high-performance lithium-ion batteries necessitates the development of cathode materials that combine high capacity, structural stability, and rapid charge–discharge capability. First-principles calculations predict that layered CrSe$_2$ possesses a robust framework capable of accommodating one Li$^+$ per formula unit while intrinsically supporting fast Li-ion diffusion. Muon spin rotation (µ$^+$SR) measurements validate this prediction, revealing fast Li$^+$ diffusion in pre-lithiated CrSe$_2$. Consistent with these findings, electrochemical testing demonstrates a reversible capacity of 125.3 mAh g$^{−1}$ at 0.1 C, approaching the theoretical value of 127.7 mAh g$^{−1}$, with stable cycling and good rate capability. In operando X-ray diffraction and electrochemical impedance spectroscopy further reveal a reversible topotactic transition and a lithiation-driven core-shell evolution during cycling. These results show that lithiation-induced conductivity changes govern the electrochemical behavior of CrSe$_2$, highlighting its potential as a high-performance cathode for LIBs. This study provides new insight into intercalation processes in layered transition-metal chalcogenides and informs the design of fast-charging electrodes.