Heavy metal, such as lead (Pb), arsenic (As), and mercury (Hg), contamination is a grave global issue that has affected public health via drinking water, paints, and a wide range of personal consumer products such as cosmetics. Here, we have used Surface enhanced Raman spectroscopy (SERS) for the novel solution phase detection of Hg2+ ions in spiked cosmetic (skin whitening) samples by using 2,5-Dimercapto-1,3,4-thiadiazole (DMcT) functionalized silver shelled iron oxide (Fe3O4@Ag-DMcT) nanoparticles (NPs). Fe3O4@Ag NPs (12 ± 4 nm) are the magneto-plasmonic SERS enhancers, and DMcT work as the Hg2+ reporter. We have optimized 2 mg/mL of Fe3O4@Ag:DMcT with 10−4 M of DMcT to be the best for SERS based Hg2+ detection in spiked samples of commercial skin whitening product. The samples mixed with the SERS probe were sealed in a capillary tube and placed on a magnet under the Raman spectroscope. A calibration curve of the variation of the 1360 cm−1 band of DMcT as a function of Hg2+ concentration was first determined using a cream, and a liquid phase randomly chosen cosmetics. Unknown samples spiked with low (10−8 M), and high (10−4 M) concentrations of Hg2+ could be successfully detected with ∼ 35 %, and 14.6 % error in measured intensities, respectively, with respect to the calibrated data. We estimate a limit of detection (LoD) for Hg2+ in real cosmetic sample as 1 nM (∼0.2 ppb).