This study compares the hydrogen-assisted cracking behavior of AISI 420 martensitic stainless steel subjected to different heat treatments and clarifies the effect of key microstructural factors, including carbides, ferrite, and residual stress. The sample treated with the quenching-ferrite/martensite duplex tempering- tempering (QDT) process exhibits the highest resistance to hydrogen embrittlement (HE), mainly attributed to its minimal microscopic residual stress and the largest amount of fine Cr23C6 carbides. Secondary cracks preferentially propagate along banded ferrite/martensite boundaries but are deflected by small ferrite particles. For the quenched sample, the fracture exhibits the intergranular cracking morphology and the dominant HE mechanism is the hydrogen-enhanced decohesion (HEDE) mechanism. For the QDT sample, the HE mechanism under the pre-charging condition is the hydrogen-enhanced plasticity (HELP) mechanism, while it transitions to the HELP mediated HEDE mechanism under the in-situ charging condition.