Background: Mercury toxicity is rare but can have severe local and systemic effects depending on the route of exposure. While mercury inhalation poses significant systemic risks, subcutaneous deposition primarily leads to localized effects, with delayed systemic absorption possible. Case Report: A 49-year-old male with a history of diabetes and mental health disorders presented with pain in his left forearm after injecting approximately 30 ml of mercury subcutaneously using an insulin syringe. Prompt imaging revealed mercury deposits in the forearm without systemic toxicity. The patient underwent two surgical washouts for mercury removal and received wound care. No complications were noted, and his condition improved during the hospital stay. Conclusion: Subcutaneous mercury injection carries a lower risk of systemic toxicity but requires early surgical removal to prevent local and systemic complications.