Health and environmental monitoring are essential for protecting ecosystems, ensuring public health, and promoting sustainable development. Nanomaterial-based electrochemical sensors have emerged as powerful tools for on-site monitoring of a wide range of analytes, including biomarkers, pharmaceuticals, heavy metals, toxic substances, and microplastics. These sensors offer rapid, reliable, and cost-effective measurements by leveraging the unique chemical and physical properties of nanomaterials, such as high effective surface area and catalytic activity, which enhance sensitivity and selectivity—critical features for effective health and environmental protection. However, traditional chemical synthesis methods for nanomaterials often involve high temperatures and hazardous chemicals, which contradict the principles of sustainability. To address these issues, green synthesis techniques have been developed, utilizing eco-friendly substances such as plant extracts, microorganisms, and other biological systems. Green synthesis not only promotes environmental sustainability and cost-efficiency but also produces nanomaterials with unique properties that can further enhance sensing performance. This chapter will review the methodologies employed in green synthesis, highlighting their distinct characteristics and the application of green nanomaterials in electrochemical sensors. It also addresses the challenges in this field and explores potential avenues for future advancements, including the integration of smart sensing technologies for real-time and remote monitoring.