Given the number of lives affected by cancer and the great potential for optimizing well-being via lifestyle changes, patients, providers, health care systems, advocacy groups, and entrepreneurs are looking to digital solutions to enhance patient care and broaden prevention efforts. Thousands of health-oriented mobile websites and apps have been developed, with a majority focused upon lifestyle behaviors (e.g., exercise, diet, smoking). In this review, we consider the use and potential of social media and mHealth technologies for cancer prevention, cancer treatment, and survivorship. We identify key principles in research and practice, summarize prior reviews, and highlight notable case studies and patient resources. Further, with the potential for scaled delivery and broad reach, we consider application of social media and mHealth technologies in low-resource settings. With clear advantages for reach, social media and mHealth technologies offer the ability to scale and engage entire populations at low cost, develop supportive social networks, connect patients and providers, encourage adherence with cancer care, and collect vast quantities of data for advancing cancer research. Development efforts have been rapid and numerous, yet evaluation of intervention effects on behavior change and health outcomes are sorely needed, and regulation around data security issues is notably lacking. Attention to broader audiences is also needed, with targeted development for culturally diverse groups and non-English speakers. Further investment in research to build the evidence base and identify best practices will help delineate and actualize the potential of social media and mHealth technologies for cancer prevention and treatment.