Mechanical degradation occurs in fuel cell membranes due to the dynamic environmental conditions of operational duty cycles, and is regarded as a critical determinant of fuel cell durability and lifetime. Imaging-based failure analysis is typically employed to characterize structural and morphological aspects of the degradation, and 3D visualization capability of X-ray computed tomography is effectively expanding the scope of this analysis. This work further leverages the additional non-destructive and non-invasive attributes of this visualization technique to capture 4D information pertaining to the evolution of mechanical degradation in fuel cell membranes. A custom fuel cell fixture is utilized to periodically track identical membrane locations during the course of its mechanical degradation, which is generated through an accelerated stress test. The predominant fatigue driven membrane crack development process is found to proceed non-linearly in time and is spatially concentrated under the uncompressed channel regions. Membrane cracking location is shown to be strongly correlated with beginning-of-life MEA defects, namely, electrode cracks and delamination. In situ crack propagation rates are quantified and the presence of a 'crack closure' effect during mechanical membrane degradation is demonstrated. Unlike crack initiation, crack propagation in the membranes does not appear to be significantly influenced by electrode morphology.