Cancer is a major cause of morbidity and mortality across the globe, with a substantial increase in cases anticipated over the next few decades. Given the constraints and adverse effects associated with standard cancer therapies, the contribution of diet and nutraceuticals to cancer prevention and treatment is receiving increased scrutiny. A diet rich in plant-based foods, extra virgin olive oil (EVOO), and bioactive compounds, including the Mediterranean Diet, has been associated with reduced cancer risk and improved treatment outcomes. This review aims to explore the complex mechanisms of the MedDiet and nutraceuticals (polyphenols, flavonoids, terpenoids) in cancer prevention, to determine their potential as cancer treatment adjuvants. Promising results show that key compounds such as bergamot polyphenolic fraction (BPF), cynaropicrin, oleuropein, quercetin, resveratrol, and serotonin can modulate oxidative stress, inflammation, the tumor microenvironment, the cell cycle, and drug resistance. A significant observation is that many of these substances demonstrate dual dose-dependent activity; they function as antioxidants in healthy cells but induce pro-oxidant and pro-apoptotic effects in cancerous cells. Their ability to boost chemotherapy's effectiveness and safety while lessening side effects and offering combined advantages is also explored. To summarize, this review suggests that the Mediterranean Diet and nutraceutical supplements may help prevent and manage cancer, but more research is needed to confirm their benefits.