To design and manufacture high-performance energy storage devices with real mechanical flexibility is one of the main advantages of the solid-state battery technology. Mechanically flexible thin film, all solid-state Li-ion batteries are supposed to be the main power sources in emerging technologies such as flexible electronics, wearables, etc. However, if a flexible solid-state device is exposed to repeated external mechanical load, introducing additional aging mechanisms might be expected. In addition, externally introduced stress and strain to the battery functional components could influence lithiation kinetics of the respective electrode material. In the present study, the effect of the external mechanical load on the lithiation kinetics and the collateral mechanical fatigue of the full battery cell during dynamic bending were investigated in detail. Therefore, mechanically flexible, all solid-state MoO3/LiPON/Li battery cells were fabricated on a polymer substrate. Battery cells were exposed to static convex bending and it was ascertained that the bulk resistance of the positive electrode is largely dependent on the depth-of-discharge as well as mechanical stress state, while other processes such as charge transfer and electrolyte bulk resistance are less affected. Furthermore, battery cells were cycled galvanostatically, while they were bent repeatedly using different bending scenarios. Below a threshold bending frequency (f = 1/360 Hz), stable battery function was found, however mechanical aging of the battery cell was observed. As it was demonstrated, the metal current collector/positive electrode interface is highly prone to the physical degradation upon dynamic bending. As a result, delamination of the electrode and contact loss occur, causing capacity fading, accordingly. The present study shed light on the joint mechanical-electrochemical aging of mechanically flexible all solid-state Li-ion batteries.