Cancer is primarily a disease of older persons. Given the heterogeneity of aging, physiological age, rather than chronological age, better expresses the cumulative effect of environmental, medical, and psychosocial stressors, which modifies life expectancy. Comprehensive geriatric assessment, a tool that helps ascertain the physiological age of older individuals, is the gold standard for assessing older adults with cancer. Several international organizations recommend using the geriatric assessment domains to identify unrecognized health problems that can interfere with treatment and predict adverse health-related outcomes, aiding complex treatment decision making. More recently, it has been shown that geriatric assessment-guided interventions improve quality of life and mitigate treatment toxicity without compromising survival. In this review, we discuss the role of comprehensive geriatric assessment in cancer care for older adults and provide the reader with useful information to assess potential treatment risks and benefits, anticipate complications, and plan interventions to better care for older people with cancer.