Green-synthesized nanocomposites represent a promising class of materials for the sensitive and selective detection of toxic heavy metals such as mercury (Hg2+), lead (Pb2+), and cadmium (Cd2+). Derived from renewable biological resources including plant extracts and biopolymers, these nanomaterials offer a sustainable alternative to conventional sensors, combining environmental safety with high-performance functionality. This chapter highlights their synthesis routes, key structural and surface properties, and applications in electrochemical, colorimetric, and fluorescence-based sensing platforms. Case studies across water, soil, and biological matrices demonstrate their real-world applicability. While challenges in reproducibility and long-term stability persist, green nanocomposites hold significant potential for next-generation portable and eco-friendly sensor technologies.