CONTEXT: Managing activities of daily living is important to people with advanced cancer or chronic obstructive pulmonary disease (COPD). Understanding disability in activities of daily living may inform service planning. OBJECTIVE: To identify the prevalence of disability in activities of daily living, associations and change over time, in older people with advanced cancer or COPD. METHODS: Secondary analysis of International Access, Rights and Empowerment (IARE) studies in adults aged ≥65 years with advanced disease in the United Kingdom, Ireland, and United States, using cross-sectional (IARE I & II) and longitudinal (IARE II, 3 timepoints over 6 months) data. Measures included disability in activities of daily living (Barthel Index), symptom severity (Palliative Outcome Scale), and assistive device use (self-reported). Logistic regression was used to identify relationships between disability and age, sex, living alone, diagnosis, and symptom burden; visual graphical analysis explores individual disability trajectories. RESULTS: One hundred fifty-nine participants were included (140 cancer, 19 COPD). Sixty-five percent had difficulty climbing stairs, 48% bathing, 39% dressing, and 36% mobilizing. Increased disability was independently associated with increased symptom burden (odds ratio, 1.08 [95% CI:1.02-1.15], P = 0.01) and walking unaided (z = 2.35, P = 0.02), but not with primary diagnosis (z = -0.47, P = 0.64). Disability generally increased over time but with wide interindividual variation. CONCLUSION: Disability in activities of daily living in advanced cancer or COPD is common, associated with increased symptom burden, and may be attenuated by use of assistive devices. Individual disability trajectories vary widely, with diverse disability profiles. Services should include rehabilitative interventions, guided by disability in individual activities of daily living.