Metallic glasses (MG’s) structures is not fully defined by its chemical composition and thermodynamic variables (absolute temperature, pressure, and volume). Identical chemical composition MG often have vastly different properties, particularly fracture toughness, depending on their thermal history. The non-equilibrium properties of MG’s are typically quantified by a fictive temperature, Tf, above which the structure of the glass is in the metastable equilibrium. We show that fracture toughness exhibits non-trivial and abrupt increase at a characteristic fictive temperature, Tfc. Further, variation of fracture toughness with Tf originates from structural changes, where linear volume (thermal) expansion changes play a negligible role. Strain rate dependence of Tf suggests a crossover of time-scales between the externally imposed time-scale and history-dependent internal plastic relaxation time-scale. This transition may be viewed as a mechanical glass transition, determining conditions where MG’s should be processed and utilized to ensure their ductility.