徳川家定
In December 1857, Nagai was appointed Kanjō-bugyō. In July 1858, Nagai (along with Iwase Tadanari) was appointed one of the first "Gaikoku bugyō" (Commissioner for Foreign Affairs). He served from August 1858 through March 1859., during which time the difficult negotiations for the Ansei Treaties took place with the United States, United Kingdom, Russia and France. In February 1859, he added "Gunkan-bugyō" to his list of titles. However, in August, following the death of Shōgun Tokugawa Iesada, Nagai was purged from office by the "Tairō" Ii Naosuke for his support of the Hitotsubashi faction over the Kishu faction for the shogunal succession.