With the increasing demand for superior performance greases in equipment, it is important to improve the efficiency of grease development. The development of multifunctional, high-performance additives is critical to improving grease development efficiency and performance. In this study, Mo2C MXene with a good crystal structure was prepared using a fluoride-free hydrothermal etching method. The tribological properties of Mo2C MXene were investigated under various test conditions, and its influence on thermal conductivity was examined. The experimental results showed that Mo2C MXene remarkably improved the antifriction, antiwear, extreme pressure, and heat transfer properties of the lithium complex grease (LCG). When the optimal additive level of 3.0 wt % is added, the average friction coefficient (AFC) and wear volume of the LCG are reduced by 27.5% and 70.8%, respectively, and the extreme pressure performance is improved by 37.5%. The lubrication mechanism of Mo2C MXene was analyzed using FIB-TEM and XPS. This is due to the integrated effect of adsorption, interlayer sliding, and chemical reaction films of Mo2C MXene on the surface of the friction partners, which enhances the tribological performance of the grease. In addition, Mo2C MXene also enhances the thermal conductivity of the LCGs. These findings offer a novel approach to developing multifunctional additives for grease.