A decline in functional status, an individual's ability to perform the normal activities required to maintain adequate health and meet basic needs, is part of normal ageing. Functional decline, however, appears to be accelerated in older patients with cancer. Such decline can occur as a result of a cancer itself, cancer treatment-related factors, or a combination of the two. The accelerated decline in function seen in older patients with cancer can be slowed, or even partly mitigated through routine assessments of functional status and timely interventions where appropriate. This is particularly important given the link between functional decline and impaired quality of life, increased mortality, comorbidity burden, and carer dependency. However, a routine assessment of and the use of interventions for functional decline do not typically feature in the long-term care of cancer survivors. This review outlines the link between cancer and subsequent functional decline, as well as potential underlying mechanisms, the tools that can be used to assess functional status, and strategies for its prevention and management in older patients with cancer.