Fadl began his career in the reign of Harun al-Rashid (r. 786–809) as a retainer of Harthama ibn A'yan, at the time head of the caliphal bodyguard. His administrative talent brought him to the attention of Harun, who appointed him a secretary in the "diwan al-kharaj" (the "Bureau of the Land Tax"). During the civil war that followed Harun's death, Fadl retired to his estates in Iraq. It was there that he came to the attention of Abu Ishaq, the younger brother of Caliph al-Ma'mun (r. 813–833) and future Caliph al-Mu'tasim, who valued is expert knowledge on taxation and agriculture. In 828, Abu Ishaq took him along to Egypt and then appointed him to head the "diwan al-kharaj".