Carbon nanomaterials such as fullerenes, carbon nanotubes (CNTs), carbon dots, graphene, and reduced graphene oxide have emerged as up-and-coming tools in colorimetric detection systems of environmental contaminants. These nanomaterials possess unique optical and electronic properties for the detectability of heavy metals, pesticides, and pathogen pollutants in water, air, and soil. Carbon nanomaterial-mediated colorimetric detection mechanisms and sensitivity, selectivity, and real-time monitoring possibilities are discussed in this chapter. Aggregation, enzymatic biomimetic, and surface functionalization of carbon nanomaterials with particular ligands or nanoparticles may lead to color changes in the absence and presence of target contaminants, thus paving an easy, fast, and inexpensive route toward environmental diagnostics. Besides, this chapter presents the engineering of multifunctional colorimetric systems, portable sensing devices, and their applications in environmental monitoring. Such nanomaterials are considered promising as soon as they are incorporated into smart and scalable technologies aimed at pollution management on a large scale and the protection of public health, as well as their role in next-generation environmental monitoring tools.