Ishii studied civil engineering at the University of Tokyo and joined the Ministry of Construction after graduating in 1981.

He left the ministry in 1992, when he was recruited into politics by the Komeito diet member Takeshi Osada.

[1][2] Ishii became chairman of the Komeito Policy Research Council in 2010 and was instrumental in building consensus for the integrated social security and tax reform adopted under the Noda Cabinet.

[1] In September 2024, Ishii became Chief Representative of Komeito, succeeding the long-serving Natsuo Yamaguchi.

The election was a setback for the ruling coalition, which lost its majority in the house, and Ishii was unexpectedly defeated despite being an incumbent party leader.