Sonoda was adopted as heir by Sawaemon Sonoda and went to Nagasaki to study in 1868. He proceeded to Tokyo the following year to study at the Daigaku Nanko, predecessor of Tokyo University.
Sonoda served in the Education Ministry and Foreign Office. In 1881 he was appointed a consul in Britain and soon resigned from the diplomatic service to enter business. Kokichi became president of the Yokohama Specie Bank and reformed it together with Shinkichi Koizumi, whom he appointed manager. Subsequently Sonoda became president of the Fifteenth Bank and the Teikoku Warehouse Company and managing director of the Nippon Railway Company and the Kobe-Kawasaki Bank. In 1918 Kokichi was created a Baron.