In this study, a Damage Precursor Index (DPI) methodology is proposed to track the evolution of fatigue damage precursors immediately after establishing the dynamic behavior of a structure. The DPI is used to measure the change in the state of fatigue damage precursors in structures exposed to vibration loads. The DPI is based on estimating the nonlinear dynamic parameters in isotropic materials prior to crack formation. The model accounts for the incubation and evolution of localized material microplasticity. Structural compliance due to the presence of the micro-plasticity is observed experimentally. The change in the dynamic response as a result of damage precursors is used to update the global dynamic parameters, which are used to calculate a corresponding DPI. The fatigue damage precursors are verified through series of macro/micromechanical characterizations of isotropic structures under vibration loads. The application of the DPI methodology to structural health monitoring systems may considerably improve health awareness in complex systems due to the addition of sensitivity to damage precursors.