Background: Maintaining functional status is important to older adults with cancer, but data are limited on how systemic treatments affect functional status. We systematically reviewed changes in functional status during systemic cancer treatments and identified characteristics associated with functional decline and improvement. Methods: We searched PubMed, Embase, Web of Science, and Cochrane Register of Controlled Trials for articles examining characteristics associated with functional changes in older adults during systemic cancer treatment published in English between database inception and January 11, 2019 (PROSPERO CRD42019123125). Findings were summarized with descriptive statistics. Study characteristics between older adult–specific and non–older adult–specific studies were compared using the Fisher exact test. Results: We screened 15,244 titles/abstracts and 519 full texts. The final analysis included 44 studies, which enrolled >8,400 patients; 39% of studies focused on older adults (1 study enrolled adults aged ≥60 years, 10 enrolled adults aged ≥65 years, and 6 enrolled adults aged ≥70 years). Almost all studies (98%) used patient-reported outcomes to measure functional status; only 20% used physical performance tests. Reporting of functional change was heterogeneous, with 48% reporting change scores. Older adult–specific studies were more likely to analyze functional change dichotomously (29% vs 4%; P=.008). Functional decline ranged widely, from 6% to 90%. The most common patient characteristics associated with functional decline were older age (n=7 studies), worse performance status (n=4), progressive disease status (n=4), pain (n=4), anemia (n=4), and worse nutritional status (n=4). Twelve studies examined functional improvement and identified 11 unique associated characteristics. Conclusions: Functional decline is increasingly recognized as an important outcome in older adults with cancer, but definitions and analyses are heterogeneous, leading to a wide range of prevalence. To identify patients at highest risk of functional decline during systemic cancer treatments, trials need to routinely analyze functional outcomes and measure characteristics associated with decline (eg, nutrition).