Abstract Understanding the multiscale mechanical behavior of clay‐rich rocks is critical for assessing fault reactivation and the long‐term integrity of underground waste repositories. This study investigates the evolution of microdeformation in Tournemire’s shale samples under compressive stress by integrating high‐resolution imaging (from an environmental scanning electron microscope) with ultrasonic P‐wave velocity measurements. Our results demonstrate that deformation initiates through shear along pre‐existing, inclined clay‐rich bands around clasts. These heterogeneities, inherited from diagenesis, control strain localization that triggers the dilatancy onset. We show that these structural changes significantly alter the magnitude and direction of P‐wave velocities, providing a direct link between microstructural damage and elastic anisotropy.