Infamous for Mad hatter syndrome and Minamata disease, mercury (Hg) is ranked high on the Agency for Toxic Substances and Disease Registry's priority list of hazardous substances for its potent neurologic, renal, and developmental toxicities. Most typical exposures are via contaminated water and food. Although regulations and advisories are exercised at various levels, Hg pollution from both natural and anthropogenic sources has remained a major public health and safety concern. Rapid detection of solvated aqueous Hg2 + ions at low levels is critical for immediate response and protection of those who are vulnerable (young children, pregnant and breast-feeding women) to acute and chronic exposures to Hg2 +. Various types of sensors capable of detecting Hg in water have been developed. In particular, the novel use of engineered carbon nanotubes (CNTs) has garnered attention due to their specificity and sensitivity towards Hg2 + detection in solution. In this focused review, we describe the sensitivity, selectivity and mechanisms of Hg2 + ion sensing at trace levels by employing CNT-based various sensor designs, and appraise the open literature on the currently applied and proof-of-concept methods. Five different types of CNT-based sensor systems are described: potentiometric, DNA-based fluorescence, surface plasmon resonance (SPR), colorimetric, and stripping voltammetric assays. In addition, the recognized merits and shortcomings for each type of electrochemical sensors are discussed. The knowledge from this succinct review shall guide the development of the next generation CNT-based biochemical sensors for rapid Hg2 + detection in the environment, which is a significant first step towards human health risk analysis of this legacy toxicant.