Iron oxychloride (FeOCl) is a unique layered material with tunable electronic properties. The conventional synthetic route of chemical vapor transition involves a thermodynamics-driven gas–solid interfacial reaction which often generates macroscopic crystals with stable facets. In this study, through analyzing the effects of the synthetic parameters on the FeOCl synthesis, we discovered the dominant contribution of the α-Fe₂O₃ precursors on the chemical property of the FeOCl product, and subsequently developed a highly-controllable synthetic route of tailoring the FeOCl structures into small sizes and exposed high-energy facets via a facile and scalable mechanical-chemical approach. The synthesized products could be systematically tuned by the ball-milling conditions of the α-Fe₂O₃ precursors. With increased milling time, the FeOCl crystallites demonstrated reduced sizes and more exposed (110) facets. Intriguingly, these small-sized FeOCl catalysts exhibited much faster Fenton-like kinetics than the pristine macroscopic FeOCl materials. Specifically, FeOCl catalysts with a 12-hour milling time showed nearly 39 times higher efficiency toward phenol degradation than the pristine FeOCl. The structure-reactivity relationship was further elucidated using the combinatory analysis via density functional theory calculation, electron paramagnetic resonance and radical quenching probe experiments. This work provides a rationale for tailoring the surface structures of FeOCl crystallites for potential applications in environmental catalysis.