Koki Watanabe studied medicine and English under Rinsho Mitsukuri and Yukichi Fukuzawa respectively.
Accompanied Envoy Tomomi Iwakura to Europe and America (1871). Upon returning was placed in charge of compiling a history of diplomatic activities. Later became director of Peers' School (1879); member of Senate (1882); Governor of Tokyo (1885); president of Tokyo University; Minister Plenipotentiary to Austria (1890); member of House of Peers. Later helped Hirobumi Ito form the Seiyukai Party.