Impact damage and fatigue are emerging challenges in the defense industry and civil infrastructure. The more pronounced material size effect induced by advanced manufacturing processes makes mechanical analysis and life prediction in these contexts more complex. Currently, there is no convenient and effective method for predicting and designing the cross-scale impact damage and fatigue performance of metal materials. This research is based on the metallic plasticity mechanisms in the impact damage and fatigue processes, investigating the material performance under the influence of the material size effect during the impact damage process. A non-local, cross-scale impact and damage constitutive theory for metallic materials was developed, and an impact damage and fatigue simulation method for advanced manufactured metals was established. This method used the conventional theory of mechanism-based strain gradient (CMSG) to describe the size effect and was built on the Johnson-Cook impact dynamics model and Lemaitre impact damage model to describe cross-scale impact dynamics and damage evolution. This approach could be conveniently implemented in finite element analysis with the VUMAT and relevant subroutines. The present work established uniaxial and U-notch bending finite element models and verified the influence of work hardening, strain rate hardening, size effect, and damage effect on static and impact dynamic response of metals. Simulation results indicated the material behavior corresponds to the material characteristic and constitutive design. The distribution and evolution of the stress, strain, strain gradient, and damage before and after material failure are also discussed. The results show that the inhomogeneous deformation caused by advanced manufacturing processes leads to higher strain gradients, which further increase the flow stress through work hardening and strain rate hardening effects and strengthen the material. However, this also causes the material to enter the damage stage earlier, leading to reduced impact and fatigue-bearing capacity or premature failure. These findings are consistent with the inherent trade-off between strength and toughness of metallic materials.