Sarcopenia and cachexia are two common and overlapping but distinct muscle wasting syndromes that predict adverse outcomes and undermine quality of life among older adults with cancer. Despite their prognostic value and negative effects on older patients’ well-being, sarcopenia and cachexia are not routinely or adequately assessed and managed in clinical oncology practice. However, efforts to recognize and manage sarcopenia and cachexia at diagnosis and during follow-up may have beneficial effects on muscle mass, physical function, and quality of life among older adults with cancer, although evidence on long-term clinical outcomes in response to targeted interventions has yet to be established. This comprehensive review attempts to (i) delineate the differences in the pathophysiology and clinical manifestations between sarcopenia and cachexia, (ii) clarify how sarcopenia and cachexia are defined in the geriatric oncology literature, (iii) describe methods for assessing sarcopenia and cachexia in clinical practice, (iv) review the prognostic value of sarcopenia and cachexia among older patients, particularly those undergoing systemic cancer treatment, and (v) discuss evidence-based strategies aimed at managing sarcopenia and cachexia for older adults with cancer.