Aim: Postoperative complications are believed to result in poorer long-term oncological outcomes. The contribution of time to chemotherapy has not been analysed in detail. Our aim was to examine the association between postoperative complications and long-term oncological outcomes and overall survival, and the influence of delay in chemotherapy on these outcomes. Method: The study was a nationwide register-based observational study that included patients undergoing surgery for colorectal cancer and receiving adjuvant chemotherapy in Denmark between 2010 and 2015. Information regarding postoperative complications was obtained from the Danish Colorectal Cancer Group national clinical registry. A Cox regression model was used to estimate disease-free survival, recurrence-free survival and all-cause mortality in patients surviving for 180 days or more after surgery. Results: A total of 4083 patients were included, of whom 690 had postoperative complications. Postoperative complications were associated with increased odds of delay to adjuvant chemotherapy (odds ratio 4.56, 95% CI 3.67–5.66, p < 0.0001). An unadjusted analysis revealed that patients with complications had poorer disease-free survival and recurrence-free survival and had increased all-cause mortality. In multivariate analysis, postoperative complications were not associated with poorer disease-free survival [hazard ratio (HR) 1.02, 95% CI 0.88–1.18, p = 0.80] recurrence-free survival (HR 1.05, 95% CI 0.89–1.25, p = 0.56) or all-cause mortality (HR 1.04, 95% CI 0.86–1.26, p = 0.67). Conclusion: This study showed no association between the occurrence of postoperative complications and long-term prognosis when adjusting for time to adjuvant chemotherapy.