Originally named as the second in line of three heirs of his father al-Mutawakkil, al-Mu'tazz was forced to renounce his rights after the accession of his brother al-Muntasir, and was thrown in prison as a dangerous rival during the reign of his cousin al-Musta'in.

Aided by the vizier Ahmad ibn Isra'il, he managed to remove and kill the leading Turkish generals, Wasif al-Turki and Bugha al-Saghir, but the decline of the Tahirids in Baghdad deprived him of their role as a counterweight to the Turks.

Faced with the assertive Turkish commander Salih ibn Wasif, and unable to find money to satisfy the demands of his troops, he was deposed and died of ill-treatment a few days later, on 16 July 869.

His reign marks the apogee of the decline of the Caliphate's central authority, and the climax of centrifugal tendencies, expressed through the emergence of the autonomous dynasties of the Tulunids in Egypt and the Saffarids in the East, Alid uprisings in Hejaz and Tabaristan, and the first stirrings of the great Zanj Rebellion in lower Iraq.

[11] Thus, when the vizier Ubayd Allah, upon being informed of al-Mutawakkil's death, went to the house of al-Mu'tazz, he did not find him there; and when his supporters, including the abna al-dawla and others and numbering several thousand, gathered in the morning and urged him to storm the palace, he refused, with the words "our man is in their hands".

[15] The new caliph was almost immediately faced with a large riot in Samarra in support of al-Mu'tazz; the rioters included not only the "market rabble" but also mercenaries from the Shakiriyya troops.

[18][19][20] Abu Ahmad played a leading role in the siege, which created a close and lasting relationship with the Turkish military, that would later allow him to emerge as the virtual regent of the caliphate alongside his brother al-Mu'tamid (r. 870–892).

The financial straits of the Caliphate had become evident already at his accession—the customary accession donative of ten months' pay for the troops had to be reduced to two for lack of funds—and had helped bring down the regime of al-Musta'in in Baghdad.

[22] The civil war and the ensuing general anarchy only worsened the situation, as revenue stopped coming in even from the environs of Baghdad, let alone more remote provinces.

[28] As a result, al-Mu'tazz refused to honor his agreement with Ibn Tahir in Baghdad, leaving him to provide for his own supporters; this led to unrest in the city and the rapid decline of Tahirid authority.

[30] In Egypt, the talented Turkish commander Ahmad ibn Tulun was appointed governor in 868 and proceeded to establish the autonomous Tulunid dynasty.

[31] In the east, Alid uprisings weakened Tahirid rule, and led to the establishment of a Zaydi state in Tabaristan, under Hasan ibn Zayd.

[30][32] Closer to home, Kharijite revolts shook the Jazira to the north, and in the south, around Basra, the first stirrings of the great Zanj Rebellion began.