Background/Objectives: Cancer remains a leading global cause of morbidity and mortality. Modifiable lifestyle factors, including diet, physical activity, weight management, tobacco and alcohol avoidance, and reduced exposure to carcinogens (e.g., ultraviolet radiation), play key roles in prevention and care. This narrative review synthesizes evidence on lifestyle-based interventions influencing cancer risk, treatment tolerance, and survivorship. Methods: A narrative review was performed in PubMed and Scopus, supplemented by manual screening via Google Scholar. Eligible publications (2001–2025) addressed diet, physical activity, obesity/weight control, carcinogen exposure (tobacco, alcohol, radiation), complementary therapies, or cancer outcomes. Priority was given to systematic reviews, randomized trials, and high-impact studies. Results: Evidence indicates that adherence to a predominantly plant-based diet, regular physical activity, and maintenance of a healthy body weight are consistently associated with reduced incidence of several cancers, including breast, colorectal, and liver cancer. Obesity and metabolic syndrome emerge as significant contributors to cancer onset and recurrence. Nutritional strategies, such as caloric restriction, ketogenic diets, and fasting-mimicking diets, have shown promise in enhancing treatment efficacy and improving quality of life. Robust epidemiology confirms that tobacco use markedly increases the risk of multiple cancers, while cessation reduces incidence and mortality; minimizing radiation exposure, particularly ultraviolet radiation through photoprotection and sun-avoidance behaviors, lowers skin-cancer risk. Complementary and mind–body therapies may alleviate treatment-related symptoms, although high-quality evidence on long-term safety and efficacy remains limited. Conclusions: Integrating lifestyle medicine into oncology offers a cost-effective, sustainable strategy to reduce cancer burden and enhance survivorship. Comprehensive programs combining dietary regulation, structured exercise, effective tobacco-cessation support, and pragmatic photoprotection/radiation-risk mitigation may extend healthspan, improve treatment tolerance, and help prevent recurrence.