Background: Older patients with colorectal cancer (CRC) experience chemotherapy dose reductions or discontinuation. Comprehensive geriatric assessment (CGA) predicts survival and chemotherapy completion in patients with cancer, but the benefit of geriatric interventions remains unexplored. Methods: The GERICO study is a randomised Phase 3 trial including patients ≥70 years receiving adjuvant or first-line palliative chemotherapy for CRC. Vulnerable patients (G8 questionnaire ≤14 points) were randomised 1:1 to CGA-based interventions or standard care, along with guideline-based chemotherapy. The primary outcome was chemotherapy completion without dose reductions or delays. Secondary outcomes were toxicity, survival and quality of life (QoL). Results: Of 142 patients, 58% received adjuvant and 42% received first-line palliative chemotherapy. Interventions included medication changes (62%), nutritional therapy (51%) and physiotherapy (39%). More interventional patients completed scheduled chemotherapy compared with controls (45% vs. 28%, P = 0.0366). Severe toxicity occurred in 39% of controls and 28% of interventional patients (P = 0.156). QoL improved in interventional patients compared with controls with the decreased burden of illness (P = 0.048) and improved mobility (P = 0.008). Conclusion: Geriatric interventions compared with standard care increased the number of older, vulnerable patients with CRC completing adjuvant chemotherapy, and may improve the burden of illness and mobility. Trial registration: ClinicalTrials.gov ID: NCT 02748811.