<p>In response to the growing demand for energy efficiency, safety, and lightweight designs in new energy vehicles, this study addresses the challenge of balancing strength and toughness in ultra-high strength steel (UHSS) components produced by hot stamping. An innovative hot stamping process based on a martensite-bainite (M-B) complex-phase microstructure was proposed to enhance the strength and toughness of UHSS. The microstructural evolution of the M-B complex-phase during the hot stamping process was investigated using commercial B1800HS steel. With a microstructure consisting of 87.4.% martensite, 8.7% lower bainite, and 3.9% film-like retained austenite by volume fraction, the steel achieved a tensile strength of 1944 MPa, total elongation of 9.66%, and impact energy of 25.8 J. Compared with conventional hot stamping, the strength of the material showed only a slight improvement, whereas elongation and impact energy were enhanced by approximately 53% and 32%, respectively. A quantitative relationship among process parameters, resulting microstructure, and mechanical properties was established. Optimization between strength and toughness was observed when the low bainite volume fraction ranged from 5.06% to 8.73%. The toughening mechanisms are attributed to the refinement of martensite lath bundles (width of 202 nm) facilitated by bainite formation, the increase in high-angle grain boundaries, and the presence of nanoscale retained austenite (width of 80 nm) between the laths. Additionally, the refinement of grains and the formation of nanoscale (Nb, Ti)C precipitates (diameter of 17 nm) further contributed to the overall strength. The developed M-B complex-phase process, which involves medium-temperature slow cooling followed by rapid low-temperature quenching, provides an optimal combination of strength and toughness for a wide range of applications.</p>