Mixed-mode fracture toughness of natural snow weak layers has rarely been measured directly, yet it is a key parameter governing dry-snow slab avalanche release on inclined, snow-covered slopes. Most prior estimates relied on assumed elastic properties of slab and weak layer rather than direct measurements. Here, we present field-based fracture experiments investigating the seasonal evolution of the mixed-mode fracture toughness of a natural weak layer composed of faceted crystals. We introduce a novel method that combines digital image correlation of snow displacement fields with inverse modeling to derive effective elastic properties of both the weak layer and the overlying slab. These experimentally constrained elastic properties provided more robust estimates of weak layer fracture toughness than previously possible. Results show a clear increase in fracture toughness over the season from 0.29 ± 0.042 J m^-2 to 0.67 ± 0.06 J m^-2 in mode I and from 0.10 ± 0.022 J m^-2 to 1.0 ± 0.1 J m^-2 in mode II. The effective elastic modulus of the weak layer evolved from approximately 0.63 ± 0.094 MPa at the beginning of the measurement period to 0.40 ± 0.046 MPa later in the season, while slab elasticity increased from 16 ± 1 MPa at the start to 40 ± 2 MPa. Furthermore, micro-computed tomography and manual snow profiles revealed that, although the weak layer density remained nearly constant, the specific surface area decreased and the mean local ice and pore thickness increased, as did the grain size. These findings suggest that the increases in both mode I and mode II fracture toughness are associated with grain size growth and growth of the bonds between the grains. The study further demonstrates that displacement-constrained inverse modeling provides robust snow elastic properties, improving fracture-mechanical estimates of avalanche release processes.