<p>Cement-based materials, as fundamental construction materials, significantly influence the advancement and quality standards of engineering projects. Traditional early-strength technologies often face challenges such as decreased durability, increased carbon emissions, and unbalanced material properties. The addition of nano-C–S–H accelerates the growth of cement hydration products, thereby markedly improving the early strength of cementitious composites. Magnesium is a ubiquitous element in cement-based materials, but its potential role in nano-C–S–H remains not fully investigated. This study innovatively introduces an external Mg²⁺ modification strategy for the preparation of nano-C–S–H and systematically studies the effects of nano-C–(M)–S–H on the early hydration kinetics and strength performance of cement. C–(M)–S–H nanocomposites with various Mg/Si ratios were synthesized via the co-precipitation method and were added into cement paste. The early hydration characteristics of cement paste were characterized using isothermal calorimetry, Q-XRD, and TG-DTG. Moreover, the compressive strength, pore structure, and micromorphology of the cement paste were tested and observed through setting time test, mechanical property test, BET, and SEM. The results indicate that nano-C–(M)–S–H can greatly speed up the early hydration of cement when the Mg/Si ratio is 0.05. Compared with nano-C–S–H, the S-0.05 showed a much shorter setting time and a larger specific surface area. At 16&#xa0;h, the compressive strength increased by 77.46% compared with the Ref. The formation of early C–S–H gel and CH is promoted, and the strength at 28d keeps increasing steadily. The external addition of Mg²⁺ in nano-C–(M)–S–H is found to be highly advantageous in enhancing the early-age performance of cement-based materials. It provides a novel idea, offering innovative perspectives for the development and application of C–(M)–S–H nano-nucleating agents.</p>