He assumed sovereignty over the Maghrib in 610 AH. Remarkable for his beauty and eloquence, he nevertheless plunged headlong into frivolity and idleness.

He had been born in 594 AH, so they enthroned him when he was but sixteen; the affairs of the ummah were thereby neglected. His mother was a concubine named Qamar al-Rūmiyya (Qamar the Roman), and he was said to resemble his grandfather.

ʿĪsā ibn ʿAbd al-Muʾmin—an uncle of his grandfather and the last surviving son of Sultan ʿAbd al-Muʾmin—presided over his investiture and lived on into the 620s AH. On the day the oath was taken, his private secretary Abū ʿAbd Allah ibn ʿAyyāsh rose and kept saying to the notables (1): “You are pledging allegiance to the Commander of the Faithful, son of the Commander (2) of the Faithful, upon the same terms on which the Companions (3) pledged allegiance to the Messenger of Allah, peace and blessings be upon him: that you will listen and obey in both ease and hardship (4).”

ʿAbd al-Raḥmān, son of al-ʿĀḍid bi-Llāh the Egyptian ʿUbaydi, then revolted against him. Having fled the Ayyubids for the Maghrib, he was joined by the Sanhāja; the calamity he caused became dire and his forces swelled. A man of dignified bearing, silence, and devotion, he marched on Sijilmāsa. Its governor—a grandson of ʿAbd al-Muʾmin—confronted him, but Ibn al-ʿĀḍid prevailed. He continued to roam, his followers ever increasing, yet none of his undertakings came to fruition: he was a stranger to the land, without kinsfolk, and his tongue was not that of the Berbers. Eventually the governor of Fes seized him and crucified him (5).

Al-Mustanṣir died in Shawwal, 620 AH, leaving no heir. After him the Almohads placed his grand-uncle ʿAbd al-Wāḥid on the throne.