Lead ions (Pb) contamination in drinking water, a major source of lead poisoning to the general population, is typically detected by bulky and costly laboratory analytical instrument. A mobile analytical device for rapid Pb sensing is a growing demand. Herein, we report smartphone nanocolorimetry (SNC) as a new technique to detect and quantify dissolved Pb in drinking water. Specifically, we have employed a single-step sedimentation approach by mixing a controlled quantity of chromate ion (CrO) to react with Pb containing solutions to form highly insoluble lead chromate (PbCrO) nanoparticles as vivid yellow precipitates. This is followed by microscopic color detection and intensity quantitation at nanoscale level using dark-field smartphone microscopy. The sum of the intensity of yellow pixels bears a highly reproducible relationship with Pb concentration between 1.37 and 175 ppb in deionized water and 5-175 ppb in city tap water. In contrast to traditional colorimetric techniques analyzing bulk color changes, SNC achieves unparalleled sensitivity by combining nanocolorimetry with dark-field microscopy and mobilized the metal ions detection by integrating the detection into the smartphone microscope platform. SNC is rapid and low-cost and has the potential to enable individual citizens to examine Pb content in drinking water on-demand in virtually any environmental setting.