Mercury (Hg) is regarded as one of the most toxic heavy metals and its exposure poses an enormous threat to the environment and human system, with widespread ecological repercussions. Mercury may enter the water bodies naturally; but anthropogenic activities particularly industrial discharges increase the Hg level. Once Hg released into aquatic environment, it undergoes multifaceted transformations via microorganisms that convert it into Methylmercury (MeHg), a potent neurotoxin, that has remarkable effect on aquatic species and lead to bioaccumulate and biomagnified to unsafe levels in food chain. Fish, being highest in aquatic food web, collect larger levels of MeHg by bioaccumulation, which induces physiological and haematological changes, damages reproductive, and respiratory system, and even affect the fish survival. In humans, the primary route of Hg exposure is through ingestion of aquatic-based food, which gets easily absorbed by gastrointestinal wall and transverse placental and blood-brain barrier, resulting in neurological, reproductive, cardiovascular, and genetic disorders. This review paper addresses the toxicological effect of Hg on aquatic organisms, and is associated with the effect of Hg toxicity on humans. It can help the researchers to assess health risks and choose the best course of action for restoration efforts of ecosystems, thereby preserving human health.