Abstract Mercury is one of the few terrestrial planets in the solar system with a magnetic field, yet the energy source to power its core dynamo remains unclear to date. Si, C, and possibly H are the top candidate light elements for its liquid core. Here, we showed experimentally that Si, C, and H in Fe‐Si‐C and Fe‐Si‐C‐H liquids share a simultaneous solubility at 9–21 GPa, 1,400°C–2,200°C, and this solubility decreases with decreasing temperature. When exceeding the simultaneous solubility, light elements will exsolve from the Fe melt as elemental phases such as diamond, graphite, silicon, and H2. We infer that if Mercury had an early liquid core primarily composed of saturated Fe‐Si‐C‐(H) liquid, subsequent cooling would lead to continuous light element exsolution in its liquid core. The gravitational potential released by the exsolved light elements can drive the compositional convection and power its long‐term core dynamo.