At the urging of Barmakid Yahya ibn Khalid, Fadl also converted to Islam, probably in 806, and entered the service of the Caliph Harun al-Rashid and his son al-Ma'mun.

After defeating al-Amin, al-Ma'mun became the new Caliph throughout the eastern Islamic world, primarily the Iranian lands, and Fadl was appointed vizier and Emir of these areas.

According to the historian al-Azraqi and Ibn Babuya, Fadl led several campaigns in Khurasan and the neighboring areas, and there the local rulers faced decisive defeats, including the Karluk Turks (whose leaders had to flee) and the Kabul Shahi.

The significance of this victory can hardly be underestimated, since Fadl not only secured the eastern flank of the empire, but also for the influx of new mercenaries and military slaves made for al-Ma'mun's army.

Most modern historians agree that it was al-Ma'mun who ordered the death of both men, despite his deep friendship and solidarity to them (with whom he was related by marriage), politics and the unity of the caliphate.