Marine bivalve molluscs, such as scallops, mussels and oysters, are crucial components of coastal ecosystems, providing a range of ecosystem services, including a quarter of the world's seafood. Unfortunately, coastal marine areas often suffer from high levels of metals due to dumping and disturbance of contaminated material. We established that increased levels of metal pollution (zinc, copper and lead) in sediments near the Isle of Man, resulting from historical mining, strongly correlated with significant weakening of shell strength in king scallops, Pecten maximus. This weakness increased mortality during fishing and left individuals more exposed to predation. Comparative structural analysis revealed that shells from the contaminated area were thinner and exhibited a pronounced mineralisation disruption parallel to the shell surface within the foliated region of both the top and bottom valves. Our data suggest that these disruptions caused reduced fracture strength and hence increased mortality, even at subcritical contamination levels with respect to current international standards. This hitherto unreported effect is important since such non-apical responses rarely feed into environmental quality assessments, despite potentially significant implications for the survival of organisms exposed to contaminants. Hence our findings highlight the impact of metal pollution on shell mineralisation in bivalves and urge a reappraisal of currently accepted critical contamination levels.