Purpose Knowledge of the contribution of high-penetrance susceptibility to familial colorectal cancer (CRC) is relevant to the counseling, treatment, and surveillance of CRC patients and families. Patients and Methods To quantify the impact of germline mutation to familial CRC, we sequenced the mismatch repair genes (MMR) APC, MUTYH, and SMAD4/BMPR1A in 626 early-onset familial CRC cases ascertained through a population-based United Kingdom national registry. In addition, we evaluated the contribution of mutations in the exonuclease domain (exodom) of POLE and POLD1 genes that have recently been reported to confer CRC risk. Results Overall mutations (pathogenic, likely pathogenic) in MMR genes make the highest contribution to familial CRC (10.9%). Mutations in the other established CRC genes account for 3.3% of cases. POLE/POLD1 exodom mutations were identified in three patients with family histories consistent with dominant transmission of CRC. Collectively, mutations in the known genes account for 14.2% of familial CRC (89 of 626 cases; 95% CI = 11.5, 17.2). Conclusion A genetic diagnosis is feasible in a high proportion of familial CRC. Mainstreaming such analysis in clinical practice should enable the medical management of patients and their families to be optimized. Findings suggest CRC screening of POLE and POLD1 mutation carriers should be comparable to that afforded to those at risk of HNPCC. Although the risk of CRC associated with unexplained familial CRC is in general moderate, in some families the risk is substantive and likely to be the consequence of unidentified genes, as exemplified by POLE and POLD1. Our findings have utility in the design of genetic analyses to identify such novel CRC risk genes.