Graduating from the National Defense Medical College in March 1990, Yamaguchi joined the Second Internal Medicine Department of the National Defense Medical College Hospital in April of the same year. After working at the Self-defense Forces Central Hospital, he joined the Saitama Cancer Center in August 1996, serving as Head Physician of the Clinical Laboratory Department, Head Physician of the Gastroenterology Department, and then Chief/Director of the Gastroenterology Department. In October 2015, Yamaguchi was appointed as Director of the Gastroenterological Chemotherapy Department, Gastroenterology Center, at The Cancer Institute Hospital of JFCR. Since then, he has also jointly served as Director of the Clinical Trial Standardization Department, Clinical Trial Development Center; Vice Director of the Clinical Trial Support Department, Clinical Trial Development Center; and Junior Hospital Director. In July 2018, Yamaguchi was appointed as Director of the Clinical Trial Development Center.
He is a member of the Gastric Cancer Treatment Guidelines Committee member; Board Certified Fellow of the Japan Gastroenterological Endoscopy Society; Provisional Trainer of the Japanese Society of Medical Oncology; member of the Board of Councilors of the Japanese Society of Medical Oncology; and part-time instructor (chemotherapy advisor) at the Digestive Tract/General Surgery Department, Saitama Medical Center.