The accumulation of methylmercury (MeHg) through the daily consumption of large predatory fish poses potential health risks. MeHg has been found to cause Minamata disease, but the full nature of MeHg toxicity remains unclear. Because of its chemical properties, MeHg covalently binds to cellular proteins through their reactive thiols, referred to as S-mercuration, resulting in the formation of protein adducts. In this review, we summarize how the S-mercuration of cellular proteins could be involved in the major mechanisms that have been suggested to underlie MeHg toxicity. Additionally, we introduce our attempts to identify cases of S-mercuration for the research to reveal the true nature of MeHg toxicity.