Tagetes erecta L. is an important industrial source of lutein and other phytochemicals with antioxidant, anti-inflammatory, and glucose- and lipid-metabolizing potential. This review analyzes its phytochemical composition, biological mechanisms, potential applications in the prevention of chronic non-communicable diseases, and emerging uses of nanotechnology to enhance biomass and secondary metabolite production. A structured search was conducted in PubMed, Scopus, Web of Science, ScienceDirect, and Google Scholar using terms related to T. erecta, carotenoids, flavonoids, metabolic health, nanoparticles, nanoelicitation, and agro-industrial valorization. The evidence indicates that lutein, zeaxanthin, quercetagetin derivatives, patuletin, quercetin, and phenolic acids can modulate oxidative stress, inflammatory signaling, carbohydrate-digesting enzymes, lipid accumulation, and cell protection. Furthermore, the flowers, leaves, and extraction residues can be used as reducing and stabilizing matrices for the sustainable synthesis of functional nanomaterials. Nanoelicitors, nanofertilizers, and nanoencapsulated systems could stimulate the methylerythritol phosphate pathway, chromoplast differentiation, and the expression of genes related to carotenoid biosynthesis. Taken together, T. erecta represents a promising platform for the development of nutraceuticals, nanobiotechnology, and circular bioeconomy applications; however, phytochemical standardization, clinical validation, mechanistic studies supported by omics sciences, and pilot-scale evaluations are still required.