Abstract: This work reveals the significant effects of cobalt (Co) on the microstructure and impact toughness of as-quenched high-strength steels by experimental characterizations and thermo-kinetic analyses. The results show that the Co-bearing steel exhibits finer blocks and a lower ductile–brittle transition temperature than the steel without Co. Moreover, the Co-bearing steel reveals higher transformation rates at the intermediate stage with bainite volume fraction ranging from around 0.1 to 0.6. The improved impact toughness of the Co-bearing steel results from the higher dense block boundaries dominated by the V1/V2 variant pair. Furthermore, the addition of Co induces a larger transformation driving force and a lower bainite start temperature (B_S), thereby contributing to the refinement of blocks and the increase of the V1/V2 variant pair. These findings would be instructive for the composition, microstructure design, and property optimization of high-strength steels.