This chapter explores the critical role of carbon-based nanomaterials in advancing electrochemical sensors for environmental monitoring, emphasizing their applications in detecting a wide range of contaminants, including heavy metals, organic pollutants, endocrine disruptors, and perfluoroalkyl and polyfluoroalkyl substances (PFAS). It begins by underscoring the importance of environmental contaminant detection and the advantages of electrochemical methods, such as high sensitivity, selectivity, and rapid response times. This chapter then provides an overview of various carbon-based nanomaterials—carbon dots, graphene, carbon nanotubes, activated carbon, and carbon-supported single-atom catalysts (CSACs)—highlighting their unique properties, such as high surface area, conductivity, and catalytic activity, which enhance electrode performance in electrochemical sensors. Focusing on performance enhancement mechanisms, the discussion covers how these nanomaterials facilitate increased contaminant detection through improved electrochemical activity and stability. Additionally, specific sensor applications are reviewed for detecting established and emerging contaminants, while key challenges in sensor design, such as stability, reproducibility, and miniaturization, are also discussed. This chapter concludes with an outlook on future advancements, discussing the potential of carbon-based electrochemical sensors for real-time environmental monitoring and effective pollution control.