Most details of the life of Jayhani are unknown. He was the son of Ahmad Jayhani, and had a brother named Ubaydallah Jayhani. Spelling patterns in his works suggests that Persian was his native language. Yaqut al-Hamawi also recorded that Jayhani frequently used the Persian expression "bedāw andarūn". Al-Muqaddasi noted that Jayhani studied philosophy, astronomy and geometry. Furthermore, he also stated that Jayhani would assemble together foreigners and ask them about the lands and the routes to get to different territories. Jayhani was a secret adherent to Manichaeism, according to Ibn al-Nadim. During his early career, Jayhani was a student of Abu Zayd al-Balkhi, and used to give him female slaves as gifts, but later left him, due to his Kitab al-Qarabin wa'l-dhaba'ih, which Jayhani disapproved. Jayhani was made guardian of Nasr II in 913. A year later, his 8-year-old ward became the ruler of the Samanid Empire and Jayhani was appointed vizier. . Due to Nasr's youth, Jayhani undertook the regency of the empire. Almost immediately a series of revolts broke out within the state, the most serious being the one led by his great-uncle Ishaq ibn Ahmad. Ishaq’s sons took part in the rebellion; one son, Abu Salih Mansur, took control of Nishapur and several other cities in Khorasan. Eventually, Ishaq was captured, while Abu SalihMansur died in Nishapur. Nasr's ascension also brought instability to the peripheries of the Samanid state. The Abbasid Caliphate managed to recover Sistan for the last time, while Ray and Tabaristan were taken by the AlidHasan al-Utrush. Despite being unable to recover the provinces, the Samanids employed numerous local Daylamite and Gilite leaders and remained active in the struggles there. After the largest mosque of Bukhara was destroyed, Jayhani financed the erection of a minaret, according to Narshakhi. Ahmad ibn Fadlan, who met him in 921, recorded that the people referred to him as "the elder bulwark", evidencing that Jayhani was still regarded as a most influential figure in Nasr II's court. A year later, however, Abu'l-Fadl al-Bal'ami succeeded him as vizier. The date of Jayhani's death is unknown. His son, Abu Ali Jayhani, was vizier of the Samanid Empire from 938 to 941. Abu Ali's son held the same office between 974 and 976.
Works
Ibn al-Nadim, who confused Abu Abdallah Jayhani with his grandson, attributed four books to Abu Abdallah. Yaqut al-Hamawi, who used al-Nadim's work as a source for his own work, even caused additional confusion by blending the three consecutive generations of the Jayhani family. According to historian István Zimonyi, the four books "point to an author experienced in politics, as every book is connected with state affairs".
''The book of Routes and Kingdoms''
Jayhani "assembled around himself some foreigners and questioned them about the countries and their revenues, the condition of roads thither, the elevation of the stars above the horizons there and the length of the meridian shadows cast by the sun", according to al-Muqaddasi.
