This study investigates heterogeneous degradation in an integrated 5‐cell proton exchange membrane (PEM) fuel cell stack subjected to a combined accelerated stress test integrating dynamic potential cycling, extended idle soaking, and passive humidification. The imposed conditions resulted in a quasi‐linear voltage degradation behaviour across operating current densities. Electrochemical impedance spectroscopy revealed a progressive increase in charge transfer resistance, while the ohmic resistance remained largely stable throughout the test, indicating the absence of severe bulk membrane failure. Post‐mortem analysis confirmed substantial platinum dissolution and redistribution within the catalyst layers, accompanied by pronounced catalyst layer–membrane interfacial delamination, particularly in inlet regions. The results demonstrate that electrochemical degradation driven by platinum loss governs the overall performance decay, while mechanically induced delamination acts as a critical accelerator by exacerbating mass transport limitations and spatial heterogeneity. These findings highlight the importance of evaluating PEM fuel cell durability under combined, realistic stressors at the stack level to capture synergistic electrochemical–mechanical degradation mechanisms relevant to practical operation.