The effectiveness of comprehensive geriatric assessment (CGA) in improving health outcomes in cancer settings is unclear. We evaluated whether CGA can improve health-related quality of life (HRQOL) in older people with cancer who are starting systemic anticancer treatment. INTEGERATE is a multicentre, open-label, pragmatic, parallel-group, randomised controlled trial that was done at three hospitals in Australia. Participants aged 70 years and older with solid cancer or diffuse large B-cell lymphoma planned for chemotherapy, targeted therapy, or immunotherapy, were randomly assigned (1:1; using a central computer-generated minimisation algorithm with a random element, balancing treatment intent, cancer type, age, sex, and performance status) to receive CGA integrated into oncology care (integrated oncogeriatric care) or usual care only. Group assignment was not concealed from the participants and clinicians. The primary outcome was HRQOL over 24 weeks, assessed at baseline, week 12, week 18, and week 24, using the Elderly Functional Index (ELFI; score range 0-100). Analyses were by intention to treat. The trial is registered with ANZCTR.org.au, ACTRN12614000399695, and is completed. Between Aug 18, 2014, and Sept 5, 2018, 154 participants were randomly assigned to integrated oncogeriatric care (n=76) or usual care (n=78). 13 participants died by week 12 and 130 (92%) of the remaining 141 participants completed two or more ELFI assessments. Participants assigned to integrated oncogeriatric care reported better adjusted ELFI change scores over 24 weeks compared with those in the usual care group (overall main effect of group: t=2·1, df=213, p=0·039; effect size=0·38), with maximal between-group differences at week 18 (mean difference in change 9·8 [95% CI 2·4-17·2]; p=0·010, corrected p=0·030, effect size=0·48). The integrated oncogeriatric care group also had significantly fewer unplanned hospital admissions at 24 weeks (multivariable-adjusted incidence rate ratio 0·60 [95% CI 0·42-0·87]; p=0·0066). No statistically significant between-group difference was observed in overall survival. CGA led to better quality of life and health-care delivery in older people receiving systemic anticancer treatment. Routine CGA-based interventions should be considered in at-risk older people starting systemic anticancer treatment. National Health and Medical Research Council (Australia), Monash University, and Eastern Health.