Liquid crystal monomers (LCMs), critical substances of liquid crystal displays in consumer electronics, are persistent pollutants, posing potential threats to marine ecosystems. Despite their bioaccumulative potential, their occurrence and possible biological impacts on marine megafauna remain understudied. We investigated LCM occurrence in Indo-Pacific humpback dolphins (Sousa chinensis) and finless porpoises (Neophocaena phocaenoides) collected from the South China Sea (2007–2021) and assessed their toxicity through in vitro assays using established dolphin cell lines. By employing robust source-tracing methodologies, we provide the first evidence that LCMs from household electronics and coastal e-waste accumulate in cetacean tissues, including blubber, muscle, and, critically, brain tissues, demonstrating blood–brain barrier penetration, a previously undocumented phenomenon of LCMs in mammalian wildlife. The temporal trend of LCM burden in porpoise blubber is correlated with shifts in global liquid crystal display production. Transcriptomic profiling revealed LCM-induced DNA damage, cell cycle arrest, and impaired cell division in cetacean cells. These findings suggest that LCMs may pose potential risks to the nervous system and other organs of marine mammals, warranting further investigation into their toxicological effects and possible implications for human health. By bridging critical gaps among everyday electronics, LCM contamination, and marine conservation, this study highlights the need for urgent regulatory actions and improved e-waste governance to mitigate ecological and public health risks.