Detection of environmental contamination such as trace-level toxic heavy metal ions mostly rely on bulky and costly analytical instruments. However, a considerable global need exists for portable, rapid, specific, sensitive and cost-effective detection techniques that can be used in resource-limited and field settings. Here we introduce a smart-phone based handheld platform that allows the quantification of mercury(II) ions in water samples with parts per billion (ppb) level of sensitivity. For this task, we created an integrated opto-mechanical attachment to the built-in camera module of a smart-phone to digitally quantify mercury concentration using a plasmonic gold nanoparticle and aptamer based colorimetric transmission assay that is implemented in disposable test tubes. With this smart-phone attachment that weighs <40 grams, we quantified mercury(II) ion concentration in water samples by using a two-color ratiometric method employing light-emitting-diodes (LEDs) at 523 and 625 nm, where a custom-developed smart-application was utilized to process each acquired transmission image on the same phone to achieve a limit of detection (LOD) of ~3.5 ppb. Using this smart-phone based detection platform, we generated a mercury contamination map by measuring water samples at over 50 locations in California (USA), taken from city tap water sources, rivers, lakes, and beaches. With its cost-effective design, field-portability and wireless data connectivity, this sensitive and specific heavy metal detection platform running on cellphones could be rather useful for distributed sensing, tracking and sharing of water contamination information as a function of both space and time.