At the time of al-Mansur's accession, most of the Fatimid mainland realm in Ifriqiya had been lost to a large-scale anti-Fatimid revolt led by the Kharijite preacher Abu Yazid, who was laying siege to al-Qa'im's fortified coastal palace city of al-Mahdiya.

Al-Mansur immediately took up the fight against the revolt with considerable energy, but kept his father's death secret until after the final suppression of the rebellion, governing instead as the ostensible designated successor and "Sword of the Imam".

[1][3] Modern historians of the Fatimid period, such as Heinz Halm and Michael Brett, suspect that al-Mansur's rise to power was the result of a palace intrigue headed by the influential slave chamberlain Jawdhar, with the participation of other figures from al-Qa'im's harem.

[3][4] His mother, Karima, and his wet nurse, Salaf, are known to have aided him in his power struggle against his half-brothers, and are described as one of few women to have participated in political affairs.

[3][6] At the time of his accession, the Fatimid Caliphate was undergoing one of its most critical moments: a large-scale rebellion under the Kharijite Berber preacher Abu Yazid had overrun Ifriqiya and was threatening the capital al-Mahdiya itself.

Already before his father's death, on 16 May, he sent by sea weapons and supplies to the besieged city of Sousse, and within days launched a coordinated attack to relieve the city: on 26 May, the garrison of Sousse, assisted by Kutama Berber cavalry from the south and troops landed by sea from the north, broke the siege and forced Abu Yazid to withdraw his forces inland, towards Kairouan.

[8] The Fatimid prince was forced to lead by example to convince the unruly Kutama to fortify the camp with a ditch and wall, since the Berbers considered this a sign of cowardice and regarded digging as slaves' work.

Abu Yazid tried to force the Fatimids to withdraw by sending his son to raid the environs of al-Mahdiya, where many of the Kutama had settled their families; but although al-Mansur sent some troops to shield them, he refused to move his main army.

[11] While al-Mansur was slowly building up his numerically inferior forces with contingents from the remote provinces of the Fatimid empire, Abu Yazid's support began to dwindle, and his followers abandoned his camp.

[13] Even in his victory dispatch to the capital, read out by the chamberlain Jawdhar, al-Mansur upheld the fiction of the still living al-Qa'im, describing himself merely as the "Sword of the Imam".

[13] The Fatimid ruler set about achieving a reconciliation with the citizens of Kairouan: he abstained from re-appointing an Isma'ili qadi over the city, instead choosing the old and respected Malikite jurist Muhammad ibn Abi'l-Manzur; he did not appoint a Kutama as governor, but a military officer of Slavic origin, Qudam al-Fata; he remitted all taxes for two years; and offered much of the captured booty as alms to the city poor.

[17] While al-Mansur remained at Kairouan, in the north al-Hasan ibn Ali al-Kalbi had gathered fresh Kutama forces at Constantine and recaptured Béja and Tunis, securing the caliph's northern flank.

[19] Following the victory, delegations from the tribes and settlements of the region began arriving at Msila to declare their loyalty to al-Mansur, including Ibn Khazar's son, Ya'qub.

[21] Barred from following Abu Yazid, al-Mansur instead turned north, to the territory of the Sanhaja Berbers, whose leader, Ziri ibn Manad, had once submitted to Caliph al-Qa'im.

[21] After giving the command to turn back east, al-Mansur fell heavily ill to a fever on 10 January 947, and remained bedridden for a fortnight.

Al-Mansur sent orders for fresh Kutama levies to be undertaken in their homelands in Lesser Kabylia, while Ibn Khazar repeated his protestations of loyalty, which included asking for the proper formula to be used in the sermon (khutba) of the Friday prayer and the coinage.

Abu Yazid's camp was captured and torched, but he managed to find refuge in the fortress of Kiyana (close to where Beni Hammad Fort was later built).

In early June, the neighbouring fortresses of Shakir and Aqqar, also held by the rebels, surrendered, and on 14 August 947, the final attack on Kiyana was launched.

[27] On the very day of Abu Yazid's death, al-Mansur declared himself as the imam and caliph, and publicly assumed his regnal title of al-Manṣūr bi-Naṣr Allāh, 'The Victor with the Help of God'.

[30] The Fatimid caliph set out from Msila on 7 September to recover the city, but when he arrived at Tahert on the 20th, he found Hamid gone: he and his followers had sailed back to Spain.

[37] Soon after, the court left the new capital, which was not yet completed, for al-Mahdiya, where al-Mansur celebrated Eid al-Fitr on 13 April for the first time as caliph, with extensive pomp.

In his sermon in the mosque, al-Mansur publicly interpreted his victory over the 'False Messiah' as a sign of divine favour, remarking that God "wishes to renew and strengthen our dynasty".

The rebels held Ibn Attaf to be too weak and passive against the Byzantines in southern Italy, allowing the latter to stop paying the agreed-upon tribute in exchange for the truce.

His mission is unclear, but likely the Byzantines were anxious to renew the truce, which was being threatened by events in Sicily, where the new dominant faction might relaunch raids against them in the name of jihad, seeking plunder.

[41] The swift restoration of Fatimid control over Sicily also appears to have brought a renewal of the truce with Byzantium for three years, and likely the payment of the arrears in tribute.

Al-Mansur supervised the preparations at Tunis in person, putting the expeditionary force of 10,500 men under the command of the Slavic eunuch Faraj (or Farakh).

[45] In May 951, the Fatimids landed near Rhegion and attacked Gerace without success, leaving after a payment of tribute, once the Byzantine relief army approached the town.

As a sign of his success, al-Hasan al-Kalbi erected a mosque in Reggio, and obliged the Byzantines to not allow any Muslim to use it and to seek refuge and asylum there.

[50] Al-Mansur's life, first as impotent heir-apparent and then as a stoically suffering ruler, was exalted in later Isma'ili teachings as an exemplar and as a sacrifice that redeemed the faithful.

As the historian Heinz Halm remarks, "if al-Mansur comes off just as badly as the other Fatimids in the books of the intransigent Malikis, the youthful victor over the terrible hordes of Abu Yazid still appears to have been popular with the inhabitants of Kairouan, and for the first time to have achieved something like loyalty towards his house".