In 751, Yakatsugu was promoted from to and appointed second assistant to the Ministry of Ceremonies. In 757, he was promoted to and appointed governor of Sagami province, in 759 the governor of Mikawa province, and in 761 the governor of Kazusa province. In 761 he was also appointed vice-envoy to Tang dynasty China, but in the next year was replaced by Fujiwara no Tamaro without making the voyage over. In 763, he was appointed vice-minister in the Shikibu-shō. Yakatsugu plotted with Fujiwara no Yoshitsugu, Ōtomo no Yakamochi, and Saeki no Imaemishi to overthrow the daijō-daijin and power of the day, Fujiwara no Nakamaro, but their plot was discovered. In 764, Yakatsugu was demoted to an assistant minister in the Dazaifu — until nine months later, when Nakamaro led his own failed rebellion. Yakatsugu was rehabilitated, promoted to, and made governor of Hitachi province. Under Dōkyō's ensuing government, Yakatsugu was steadily promoted, rising in 765 to and lieutenant general in the imperial guard, and joining the ranks of the kugyō with a promotion to sangi in the next year, 766. Later that year, he was promoted to, and in 768 to. Upon the death of Empress Shōtoku in 770, Yakatsugu aligned with Fujiwara no Nagate in supporting the futureEmperor Kōnin as her successor. The appreciative new emperor continued to value him, and Yakatsugu was promoted to chūnagon in 771 and dainagon in 780, making him the third highest ranking figure in the daijō-kan after udaijinŌnakatomi no Kiyomaro and naidaijin Fujiwara no Uona. During this period, he also held positions as head of the Dazaifu, shikibu-sho, Ministry of the Center, and Crown Prince's Quarters. In 775, he reclaimed his family's old name of Mononobe no Ason, but in 779 he changed it back to Isonokami, now as. In 781 Yakatsugu was promoted to, but died two months later. He was posthumously awarded the rank of.
Personality
Intelligent and quick on the uptake, Yakatsugu also possessed a fine appearance. His speech and behavior were calm and elegant. A great lover of the Confucian classics and historical texts, he was familiar with a wide variety of books. He also liked to write, and was proficient in both cursive script and clerical script. He wrote kanshi, and was considered a leading scholar of his day along with Ōmi no Mifune. His works are included in the Keikokushū. He was also versed in Buddhism, and wrote and on the subject. He built a temple called Ashuku-ji on his former residence, and in a corner of the complex he established a library called. Here he made various texts available to the public, focusing primarily on non-Buddhist works. This is believed to have been Japan's first public library.
