Initial tool wear exerts a substantial influence on tool life and machining accuracy, yet this subject remains comparatively under-explored in comparison to the steady wear stage. The present study proposes a novel thermomechanical wear prediction model for the worn tool flank face during the initial wear stage, which is grounded in the tri-zone phenomenon. The model analytically describes the spatial distributions of sliding velocity, normal stress, and interface temperature along the tool–workpiece interface, highlighting their critical roles in wear evolution. The experimental observations confirmed the presence of a tri-zone phenomenon, comprising sticking, transition, and sliding zones, on the worn flank face. Velocity gradients are captured using analytical functions, while normal stress is derived from Waldorf's slip-line theory, incorporating characteristic length parameters. A one-dimensional steady-state thermal model is constructed with both shear and frictional heat sources, and jointly calibrated with simulation data. Subsequently, a calibrated Usui wear rate model is implemented, and the nodal displacement method is employed to simulate the evolution of flank wear morphology. The predicted average interface temperature in each region deviates by less than 3%, and cutting tests on Inconel 718 using TiAlN-coated tools confirm an overall wear prediction error within 6.24%. Multi-scale SEM and EDS analyses validate adhesion as the dominant wear mechanism and reveal progressive coating degradation, providing microstructural support for the proposed model. This study proposes a novel framework for mechanistic wear analysis and predictive modeling of tool life during the initial wear stage.