Abstract Background The aim of this study was to identify predictors of mortality in elderly patients undergoing colorectal cancer surgery and to develop a risk score. Methods This was an observational prospective cohort study. Individuals over 80 years diagnosed with colorectal cancer and treated surgically were recruited in 18 hospitals in the Spanish National Health Service, between June 2010 and December 2012, and were followed up 1, 2, 3, and 5 years after surgery. Sociodemographic and clinical data were collected. The primary outcomes were mortality at 2 and between 2 and 5 years after the index admission. Results The predictors of mortality 2 years after surgery were haemoglobin ≤ 10 g/dl and colon locations (HR 1.02; CI 0.51–2.02), ASA class of IV (HR 3.55; CI 1.91–6.58), residual tumour classification of R2 (HR 7.82; CI 3.11–19.62), TNM stage of III (HR 2.14; CI 1.23–3.72) or IV (HR 3.21; CI 1.47–7), LODDS of more than − 0.53 (HR 3.08; CI 1.62–5.86)) and complications during admission (HR 1.73; CI 1.07–2.80). Between 2 and 5 years of follow-up, the predictors were no tests performed within the first year of follow-up (HR 2.58; CI 1.21–5.46), any complication due to the treatment within the 2 years of follow-up (HR 2.47; CI 1.27–4.81), being between 85 and 89 and not having radiotherapy within the second year of follow-up (HR 1.60; CI 1.01–2.55), no colostomy closure within the 2 years of follow-up (HR 4.93; CI 1.48–16.41), medical complications (HR 1.61; CI 1.06–2.44), tumour recurrence within the 2 years of follow-up period (HR 3.19; CI 1.96–5.18), and readmissions at 1 or 2 years of follow-up after surgery (HR 1.44; CI 0.86–2.41). Conclusion We have identified variables that, in our sample, predict mortality 2 and between 2 and 5 years after surgery for colorectal cancer older patients. We have also created risks scores, which could support the decision-making process. Trial registration ClinicalTrials.gov, NCT02488161.