Background: This study investigated the effect of comorbidity, age, health insurance payer status, and race on the risk of patient nonadherence to NCCN Clinical Practice Guidelines in Oncology (NCCN Guidelines) for Colon and Rectal Cancers. In addition, the prognostic impact of NCCN treatment nonadherence on overall survival was assessed. Patients and Methods: Patients with CRC who received primary treatment at Memorial University Medical Center from 2003 to 2010 were eligible for this study. Modified Poisson regression was used to obtain risk ratios for the outcome of nonadherence with NCCN Guidelines. Hazard ratios (HRs) for the relative risk of death from all causes were obtained through Cox regression. Results: Guideline-adherent treatment was received by 82.7% of patients. Moderate/severe comorbidity, being uninsured, having rectal cancer, older age, and increasing tumor stage were associated with increased risks of receiving nonadherent treatment. Treatment nonadherence was associated with 3.6 times the risk of death (HR, 3.55; 95% CI, 2.16–5.85) in the first year after diagnosis and an 80% increased risk of death (HR, 1.80; 95% CI, 1.14–2.83) in years 2 to 5. The detrimental effect of nonadherence declined with increasing comorbidity and varied according to age. Conclusions: Although medically justifiable reasons exist for deviating from NCCN Guidelines when treating patients with colorectal cancer (CRC), those who received nonadherent treatment had much higher risks of death, especially in the first year after diagnosis. This study’s results highlight the importance of cancer health services research to drive quality improvement efforts in cancer care for patients with CRC.