Nanotechnology has made strides towards sustainable methods that combine environmental friendliness with effective performance. Among the green synthesis options, citrus fruits have proven to be very useful bioresources. They are rich in phytochemicals like flavonoids, phenolic acids, terpenes, and alkaloids, which work as reducing, stabilizing, and directing agents. This review focuses specifically on how citrus-derived phytochemicals help control the formation, growth, and surface features of nanostructures. It compares the synthesis of metal, metal oxide, and mixed metal oxide nanoparticles, paying special attention to the shift from single-component systems to binary and composite nanostructures. The review discusses the link between phytochemical composition and nanoparticle properties such as size, shape, and functionality. Furthermore, it examines the dual uses of these biogenic nanomaterials in medical and environmental fields. In medical applications, citrus-mediated nanoparticles show strong antibacterial, antioxidant, anticancer, and antidiabetic effects, emphasizing their potential in therapy and drug delivery. In environmental applications, their effectiveness in photo-catalysis, dye degradation, and wastewater treatment is analyzed in relation to performance and sustainability. By looking at synthesis methods, structure-property relationships, and application-driven performance, this review provides a clear view of citrus-based nanotechnology. It also points out important challenges, including scalability, reproducibility, and understanding of mechanisms, that need to be tackled to move from lab-scale studies to practical applications.