Kammala jumla mai zuwa: Bayan mutuwar Harun a Khurasan a farkon 809, Abdallah ya kasance a lardin, ya shiga kotun magajin Harun na biyu, al-Ma'mun (r. 813833), a cikin Marw. A cikin
yakin basasa tsakanin al-Ma'mun da dan uwansa, Khalifa al-Amin (r. 809<unk>813), ya kasance a gefe a Marw. Ya kasance daya daga cikin shugabannin sojoji da yawa wadanda suka ki jagorantar gwamnatin al-Ma'mun, wani mukami wanda daga karshe ya fada hannun Fadl ibn Sahl. Dangantakarsa da al-Ma'mun ba ta da kyau, amma ya bi shi zuwa yamma kuma yana tare da shi lokacin da ya shiga Baghdad a 819.