The two most common inherited forms of colorectal cancer (CRC) are familial adenomatous polyposis and hereditary nonpolyposis colorectal cancer. They are caused by germline mutations in the APC and mismatch repair genes, respectively. The simultaneous inheritance of both an APC gene mutation and a mismatch repair gene mutation is very rare. In this study, we reported a coinheritance of mutations in the APC and MLH1 genes in a 20-year-old patient with CRC and multiple polyposes. We described for the first time the somatic mutational profile of a colorectal tumor harboring concurrent germline mutations in the APC and MLH1 genes.