Increased effort for lightweighting of automotive structures results in the replacement of traditional metals with composites. Adhesive bonding is the best joining solution for composite components owing to its superior stress distribution and lightweighting. However, adhesive and adhesive-adhered interfaces are the weakest links in the structure. During fatigue crack propagation, joints do not show any significant visual changes. Thus, fatigue damage in adhesive bond lines is one of the challenging and complex failure mechanisms that requires real-time diagnostic and prognostic techniques to avoid any catastrophic failure. This paper proposes an acoustic technique for real-time fatigue damage diagnosis and prognosis. Based on experimental guided wave modal analysis, symmetric mode at 85 kHz is found to be the most sensitive mode–frequency combination for fatigue monitoring of selected lap-joint specimens. Further, a hybrid data-driven damage propagation model is used to estimate the remaining useful life in the bond line. The developed techniques were successfully implemented and validated on a single lap joint under fatigue loading. Estimated damage levels and remaining useful life are in good agreement with reference measurements. Successful validation is an indicator of the potential application of this technology in automotive industries.