Ja'far's prominence increased during the reign of al-Ma'mun's successor al-Mu'tasim (), who employed him in several of the campaigns of his caliphate. In 837 he and Itakh were sent by the caliph to reinforce al-Afshin against the rebel Babak Khorramdin in Adharbayjan, and he played a leading role in the effort to take Babak's stronghold of al-Badhdh. In the following year he was placed in command of the caliph's left wing during the Amorium Campaign against the Byzantines, and during the assault against Amorium itself he was assigned a sector of the city to attack. In 839 he received the governorship of the Yemen and appointed deputy governors to enforce his rule in that province, but in 840 he incurred the anger of the caliph, who dismissed him from his position and briefly imprisoned him in the custody of Ashinas.