A 7 1-year-old man presented to our hospital with constipation and abdominal pain. Computed tomography of the abdomen and colonoscopy revealed advanced cancer of the transverse colon. The biopsy specimen indicated a highly differentiated adenocarcinoma. The patient underwent extended right hemicolectomy with regional lymph node dissection. Pathological examination showed a neuroendocrine carcinoma (NEC) with concurrent adenocarcinoma of the transverse colon and regional lymph node metastases of the NEC and adenocarcinoma. The histopathological examination confirmed a diagnosis of mixed adenoneuroendocrine carcinoma (MANEC) in accordance with the 2010 WHO Classification of Tumors of the Digestive System. Liver and lung metastases were identified 8 months after the surgery. We administered chemotherapy including 5-fluorouracil, Leucovorin, and oxaliplatin (mFOLFOX) plus bevacizumab, with limited therapeutic effect, as the disease progressed despite treatment. The patient chose best supportive care 13 months after the surgery. Several studies have reported that most patients with adenoendocrine cell carcinoma, including MANEC, experience relapse within 1 year after surgery, and few patients remain disease-free for long periods after surgery. The optimal strategy for the management of MANEC is variable owing to its rarity; only 2 cases of MANEC in the colon, including the present case, have been reported in Japan. It is thus important to gather more evidence on this disease and its management.