Abstract: Impact damage and fatigue are emerging challenges in defense industry and civil infrastructure, including aerospace and rail transportation. The application of advanced manufacturing processes has further complicated the mechanical analysis and lifespan study in these contexts. Currently, there is no convenient and effective method for predicting and designing the cross-scale impact damage and fatigue performance of metals, which could also account for the multi-scale microstructure features of metallic materials. The present paper, grounded in the plastic mechanics of metals during impact damage and fatigue processes, investigates the material constitutive behavior under the influence of material size effects. It establishes a delocalized, cross-scale impact constitutive relationship and model for metallic materials, and results in a convenient numerical simulation method for the impact damage and fatigue of multi-scale microstructured metals aimed at advanced manufacturing. This novel method uses the conventional theory of mechanism-based strain gradient theory (CMSG) to describe the material size effect, while simultaneously incorporating impact dynamic models such as the Johnson-Cook model and a modified Lemaitre damage model. This approach is readily implemented within the VUMAT subroutine for finite element simulations. The numerical results show that the proposed constitutive model can effectively capture the mechanical response of metallic materials exhibiting microstructural size effects under impact and multiple impact loads. Moreover, finite element simulations indicate that the heterogeneous deformation induced by the material microstructure leads to higher strain gradients, further amplifying the rate-hardening effect during the impact process. This, in turn, increases the stress and strain level of the material under impact loading, driving it into the damage stage more quickly, which ultimately reduces the material’s load-bearing capacity or causes premature failure. These findings are consistent with the inherent trade-off between strength and toughness in metallic materials.