Abstract: Increasing the carbon content in low-alloy steels is one of the most cost-effective and efficient methods for enhancing strength, often resulting in a significant reduction in ductility. In this study, a high-carbon low-alloy steel with a tensile strength of about 2.6 GPa and a total elongation of 12% was developed, achieving through the synergistic applications of two key strategies: i) refine prior austenite grains (PAGs) leading to the transition of quenched microstructure from brittle twinned martensite to dislocation martensite; ii) suppress the martensitic transformation finish temperature to sub-room temperature by the combined effect of high content of carbon and alloying elements, i.e., Ni, Mn, Si, Cr, and Mo. After quenching and tempering, the steel retains approximately 15vol% stable retained austenite (RA), which enhances ductility through the transformation-induced plasticity (TRIP) effect. These strategies collectively contribute to both high strength and excellent ductility, enhancing the strength–ductility synergy in ultra-high strength steels.