This study examined the controlling factors for the quenching crack sensitivity of ultra-strong martensitic steel with a tensile strength exceeding 1.8 GPa. Two factors, the content of carbon alloyed in the steel and the type of quenchant used in the quenching process, were evaluated in terms of the strain level and diffusible hydrogen concentration, which were measured by electron backscattered diffraction-kernel average misorientation and thermal desorption spectroscopy, respectively. This study demonstrated that specimens with a higher carbon content exhibited larger lattice distortion and a higher dislocation density during the quenching process and may be more susceptible to cracks propagating along the prior-gamma grain boundaries. The decrease in quenching rate and diffusible hydrogen concentration can be effective technical strategies for improving the mechanical toughness and mitigating the quench cracking of ultra-strong steels with a tensile strength of 2.0 GPa.