Bivalve shells, like other hard external armors, protect not just from single, potentially shattering forces, but also from a lifetime of threats. Even non-lethal forces can weaken and damage shells through the process of fatigue. The long-term danger posed by these threats depends on the animal’s capacity for repair, and their ability to maintain repair against ongoing threats. Therefore, we tested the ability of California mussels (Mytilus californianus) to repair damage from chronic fatiguing forces. Every week for seven months, we compressed live mussels for 15 cycles at a moderate force (~50% of shells’ predicted one-time breaking force). We measured the final strength as well as the initial and final morphology of the fatigued shells and a group of non-fatigued control shells that were maintained under the same intertidal-simulating lab conditions. Despite months of fatigue, the fatigued and control shells did not differ in final strength. However, the fatigued shells were significantly thicker and had significantly more internal repair. These findings suggest that even a nonlethal fatiguing force, when chronic, can be sufficient to trigger a repair response, and that this response is sufficient to maintain shell strength. Maintenance of strength had consequences for shell shape, however. Fatigued shells showed significantly less increase in width compared to non-fatigued shells. In summary, mussels were able repair and maintain strength in response to ongoing mechanical threats, which is likely important to their success on exposed rocky coasts, surviving failed predator attacks and wave- and debris-caused damage. However, the long-term consequences of morphological changes associated with chronic damage and repair require further investigation.