Koichi Yamamoto left Kogyo Senshu (Technical Study) Middle School without graduating (1928).
Koichi Yamamoto joined Ronoto (Labor Farmer Party) (1929). He became chief secretary of Tokyo District Committee of Tenants' Union and chief secretary of Gifu Prefectural League of Shakai Taishuto (Social Masses' Party) (1932) and chief director of Gifu Prefectural Barley-Cleaning Industry Union (1939) and then served successfully as chairman of Gifu Prefectural Union of Firewood Producers, chief secretary of Gifu Prefectural League of Left-Wing Socialist Party, member of Executive Committee of Japan Farmers Union, chairman of Gifu Prefectural League of Sodomei (Japan Federation of Labor Unions) and member of Central Executive Committee of Left-Wing Socialist Party.
Koichi Yamamoto became member of Control Committee of Left-Wing Socialist Party and auditor of Japan Farmers' Union (1951). Re-elected for third time to House of Representatives (1953) and served as member of Steering Committee and Standing Committee for Agriculture and Forestry. He became member of Central Executive Committee of Left-Wing Socialist Party and director of Young Men & Women's Bureau (1954).