Present work demonstrates the operation of a smartphone platform sensor that accurately measures mercury (Hg (II)) level concentration in water. The sensing principle of the designed sensor is based on the detection of fluorescence emission from a ternary complex containing different concentration of Hg (II) in the mixture. The designed sensor correlates the level of Hg (II) concentration in a given sample through the emission of fluorescence emission from it. Using simple optical components, a compact optical set-up has been designed which can be attached to the rear-camera of the smartphone. Using the designed smartphone sensor Hg (II)-level variation as low as 50ppb can be detected accurately and reliably. The performance of the sensor has been evaluated in presence of other interfering elements such as iron, copper, zinc and manganese and we noticed that the sensor characteristic does not perturb by the presence of such elements. It has been observed that the proposed sensor performs at par with that of a laboratory grad optical spectrometer. Owing to the involvement of low-cost components and user-friendly application that essentially converts the CMOS illumination reading into a readable form we envision that the proposed sensing technique would be useful for in-field based sensing of Hg(II) level in water largely present in industrial waste water and other environmental water bodies.