The impact of cycle completion rates of oxaliplatin-based adjuvant chemotherapy for stage III colon cancer in real-world practice is unknown. We assessed its impact, and that of treatment modification, on 3-year cancer-specific mortality. Four thousand one hundred and forty-seven patients with pathological stage III colon cancer undergoing major resection from 2014 to 2017 in the English National Health Service were included. Chemotherapy data came from linked national administrative datasets. Competing risk regression analysis for 3-year cancer-specific mortality was performed according to completion of <6, 6-11, or 12 5-fluoropyrimidine and oxaliplatin (FOLFOX) cycles, or <4, 4-7, or 8 capecitabine and oxaliplatin (CAPOX) cycles, adjusted for patient, tumour and hospital-level characteristics. Median age was 64 years. Thirty-two per cent of patients had at least one comorbidity. Forty-two per cent of patients had T4 disease, and 40% had N2 disease. Compared to completion of 12 FOLFOX cycles, cancer-specific mortality was higher in patients completing <6 cycles [subdistribution hazard ratios (sHR) 2.17; 95% CI 1.56-3.03] or 6-11 cycles (sHR 1.40; 95% CI 1.09-1.78) (P < .001). Compared to completion of 8 CAPOX cycles, cancer-specific mortality was higher in patients completing <4 cycles (sHR 2.02; 95% CI 1.53-2.67) or 4-7 cycles (sHR 1.63; 95% CI 1.27-2.10) (P < .001). Dose reduction and early oxaliplatin discontinuation did not impact mortality in patients completing all cycles. Completion of all cycles of chemotherapy was associated with improved cancer-specific survival in real-world practice. Poor prognostic factors may have affected findings, however, patients completing <50% of cycles had poor outcomes. Clinicians may wish to facilitate completion with treatment modification in those able to tolerate it.