Like other rigid armors, bivalve shells protect from potentially lethal predatory and environmental threats that range in frequency and magnitude from single powerful predator strikes to repeated insults from ocean waves. Shells’ effectiveness at defending from such forces is often quantified with a test of one-time breaking stress (strength): a shell is compressed until it breaks. However, this technique cannot reveal how shells resist mechanical fatigue, a process by which repeated, subcritical stresses weaken and break a material. Furthermore, the long-term threat posed by fatigue hinges on the animal’s capacity for repair. We quantified and contextualized fatigue resistance and repair in the California mussel (Mytilus californianus) to identify the ecological threat of fatigue. We used two tests of fatigue resistance: applying a subcritical load constantly or cyclically until fracture. Mussel shells broke when fatigued such that lower forces required longer loading periods before fracture. We also measured the ability of live mussels to repair non-lethal fatigue damage (15 cycles at 67% of predicted strength) over one month. Shells were weakened by fatigue but, on average, repaired within one week. Strong predators can fracture shells with a single impact, and low forces (e.g., a shell clamping shut) won’t cause damage on ecologically relevant timescales. Fatigue can make intermediate forces a threat, though; weaker predators can fatigue otherwise inaccessible prey, and failed predation attempts and episodic threats (e.g., hurled debris) can weaken shells. However, mussels have the capacity for speedy repair. A mussel would have little recourse during one predator attack, but one week between threats is sufficient for repair. Rapid, repeated forces can break shells, but, if survived, do not cause irrecoverable lifelong damage.