In 1890, Yamaguchi was appointed to the 2nd Bureau of the General Staff Office.[2] He was promoted to major in 1891, and was appointed an instructor at the Military Medical School in 1893.[3] The following year, he was appointed 2nd Battalion Commander of the 21st Infantry Regiment[4] and fought in the First Sino-Japanese War.
On October 11, 1897, Yamaguchi was appointed colonel of the Army Infantry, principal of the Toyama Military Academy, and director of the Kangunbu (a management organization of the Imperial Japanese Army).[5] The next year, he was appointed Chief of Staff, 11th Division.[6]
Yamaguchi was made major general and appointed commander of the 5th Infantry Brigade on May 5, 1902.[1] However, during the Russo-Japanese War, Nanbu Shinpei replaced Yamaguchi as acting commander in 1904. Yamaguchi was officially dismissed from the post[7] and sent to temporary retirement in January 1905. In March 1907, he was placed in service in the first reserve.[1]
He received the court rank of Junior Fourth Rank[8] on April 20, 1907.[9]