He assumed the caliphate after his brother al-Mustaʿṣim, son of al-Mustanṣir. Such a succession had not occurred since the days of al-Muqtafī ibn al-Mustazhir, for he, too, had taken power after his brother al-Rashīd ibn al-Mustarshid ibn al-Mustazhir—and Allah knows best.

His reign lasted, until his disappearance—as will be explained—five months and twenty days, making it the briefest of all the Abbasid caliphates.

As for the Umayyads (1), the reign of Muʿāwiyah ibn Yazīd ibn Muʿāwiyah lasted forty days (2); Ibrāhīm ibn al-Walīd ibn Yazīd, called al-Nāqiṣ, ruled for seventy days (3); and his brother Yazīd ibn al-Walīd held the caliphate for five months (4).

The caliphate of al-Ḥasan ibn ʿAlī after his father endured seven months and eleven days, while Marwān ibn al-Ḥakam reigned nine months (5) and ten days.

Among the Abbasids (6) there were likewise caliphs who did not reach a full year: al-Muntaṣir ibn al-Mutawakkil, whose tenure was six months, and al-Muhtadī ibn al-Wāthiq, whose tenure was eleven months and a few days (7).

The caliph al-Mustanṣir was quartered in the Mountain Citadel, in a tower together with his household and servants. On the seventh of Rajab he rode out in the black regalia, came to the congregational mosque within the Citadel, ascended the pulpit, and delivered a sermon in which he praised the noble lineage of the Abbasids. He opened by reciting the opening verses of Surah al-Anʿām, invoked blessings upon the Prophet ﷺ, voiced satisfaction with the Companions, and prayed for Sultan al-Ẓāhir. He then came down and led the congregation in prayer, an act the people greatly admired. It was a blessed hour and a day to be remembered.