A native of Nakaminato (now Hitachinaka), Ibaraki and he was raised in Hitachi until his graduation from public senior high school.

Komeito suffered a major defeat in the 2009 general election, losing ten seats, including party president Akihiro Ota and secretary general Kazuo Kitagawa.

He had been chosen through a process centered on party heavyweights Takenori Kanzaki and Chikara Sakaguchi.

After fifteen years leading Komeito, Yamaguchi stepped down in September 2024 and was succeeded by Keiichi Ishii.

[4] Yamaguchi has come out in support of the Selective Surname System for married couples and same-sex marriage,[5][6] as well as voting rights for permanent foreign residents.