After graduating from the faculty of law of the University of Tokyo in 1978, he joined the Ministry of Foreign Affairs in 1979 and received a Ph.

D from SAIS at Johns Hopkins University in the United States while an attache to the Embassy of Japan in Washington, D.C.

He was an ardent admirer of Ronald Reagan, who was serving as U.S. President during Yamaguchi's student years in Washington, D.C. Yamaguchi served in several bureaus of the Ministry of Foreign Affairs (MOFA) in Tokyo, as well as the Japan Defense Agency during the period 1982 to 1989.

[1] He studied for his master's degree at SAIS in Washington, D.C. beginning in the fall of 1980 through the spring of 1982 and completed his doctoral dissertation and received his PhD thereafter.

A strong interest of Yamaguchi's that figured prominently in his master's thesis and doctoral dissertation was the evolution of U.S.-Japanese relations in the modern era.