Grain refinement of the parent austenite led to a significant change in the intervariant boundary network of martensite, ultimately improving the mechanical properties (i.e., toughness and hardness). Molecular dynamics simulation demonstrated that the propensity for crack propagation was largely governed by the intervariant boundary energy, where the crack propagation rate was much faster for the high-energy 60 deg/[110] twist boundary than the low-energy 60 deg/[111] symmetric tilt boundary. This agreed with experimental observations where parent austenite grain refinement increased the low-energy boundary population at the expense of high-energy intervariant boundaries in martensite. In turn, this led to a significant toughness improvement without sacrificing the strength. This finding demonstrates that the mechanical properties of a martensitic microstructure can be significantly improved through intervariant boundary network engineering.