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Continuous discoveries in the field of metallic conductive MXenes have shown their feasibility as electrode materials, but their employment remains impeded by low surface area and inhomogeneous edge terminations generated by hazardous HF etching. To solve these problems, for the first time, a eutectic mixture etching strategy is utilized to accomplish one‐step synthesis of Cl‐terminated MXene (Ti3C2Cl2) with tunable in‐plane porosity from a MAX precursor (Ti3AlC2) through manipulating the phase transition of the selected salt melt. Specifically, the temperature and composition of the NaCl/ZnCl2 salt mixture are controlled to initiate a mechanism that creates and critically preserves the MXene pore structure, leading to substantial increment in material mesoporosity and a fourfold increase in surface area. Moreover, X‐ray spectroscopy analyses reveal increased TiC6 octahedral symmetry and density functional theory (DFT) modeling suggests a lower Li diffusion barrier, which imply high suitability for ion transport. Benefiting from these optimizations, mesoporous Ti3C2Cl2 delivers a high capacity of 382 mAh g−1 at 0.1 A g−1 as a dual‐ion battery anode, with capacity retention over 89% after 1000 cycles at 2.0 A g−1. Overall, this study presents a green chemistry approach that enables direct synthesis of MXenes with optimal porosity and surface termination for electrochemical applications, providing fresh insights for targeted structure modifications. [ABSTRACT FROM AUTHOR]