The data in this sheet present magnesium isotopic composition obtained from micro-drilled powder samples of cold-water corals analyzed by multiple collector inductively coupled plasma mass spectrometry (MC-ICP-MS) and corresponding elemental compositions by laser ablation inductively coupled plasma (LA-ICP-MS). Micro-drilling was performed across distinct growth stages in three specimens to investigate how Mg isotopic variations reflect magnesium incorporation mechanisms in different growth structures. Sampling position represents each measurement point's distance relative to the growth central axis.