Al-Mu'tadid was the son of al-Muwaffaq, who was the regent and virtual ruler of the Abbasid state during the reign of his brother, Caliph al-Mu'tamid. As a prince, the future al-Mu'tadid served under his father in various military campaigns, most notably in the suppression of the Zanj Rebellion, in which he played a major role. When al-Muwaffaq died in June 891 al-Mu'tadid succeeded him as regent. He quickly sidelined his cousin and heir-apparent al-Mufawwad, and when al-Mu'tamid died in October 892, he succeeded to the throne. Like his father, al-Mu'tadid's power depended on his close relations with the army, first forged in the campaigns against the Zanj and reinforced in later expeditions which the Caliph led in person: al-Mu'tadid would prove to be the most militarily active of all Abbasid caliphs. Through his energy and ability, he succeeded in restoring to the Abbasid state some of the power and provinces it had lost during the turmoils of the previous decades.
In a series of campaigns he recovered the provinces of Jazira, Thughur and Jibal, and effected a rapprochement with the Saffarids in the east and the Tulunids in the west that secured their—albeit largely nominal—recognition of caliphal suzerainty. These successes came at the cost of gearing the economy almost exclusively towards maintenance of the army, which resulted in the expansion and rise to power of the central fiscal bureaucracy and contributed to the Caliph's lasting reputation for avarice. Al-Mu'tadid was also renowned for his cruelty when punishing criminals, and subsequent chroniclers record his extensive and ingenious use of torture. His reign also saw the permanent move of the capital back to Baghdad, where he engaged in major building activities. A firm supporter of Sunni traditionalist orthodoxy, he nevertheless maintained good relations with the Alids, and was interested in natural sciences, renewing caliphal sponsorship of scholars and scientists.
Despite his successes, al-Mu'tadid's reign was ultimately too short to effect a lasting reversal of the Caliphate's fortunes, and the revival that he spearheaded was too dependent on the presence of capable personalities at the helm of the state. The brief reign of his less able son and heir, al-Muktafi, still saw some major gains, notably the annexation of the Tulunid domains, but his later successors lacked his energy, and new enemies appeared in the form of the Qarmatians. In addition, the factionalism within the bureaucracy, that had become apparent during the later years of al-Mu'tadid's reign, would debilitate the Abbasid government for decades to come, eventually leading to the subjugation of the Caliphate to a series of military strongmen and culminating in the conquest of Baghdad by the Buyids in 946.
Al-Mu'tadid was born Ahmad, the son of Talha, one of the sons of the Caliph al-Mutawakkil (r. 847–861), and a Greek slave named Dirar. The exact date of his birth is unknown; as he is variously recorded as being thirty-eight or thirty-one years old at the time of his accession, he was born around either 854 or 861. In 861, al-Mutawakkil was murdered, beginning a period of internal turmoil, known as the "Anarchy at Samarra" from the site of the Caliphate's capital, which ended in 870 with the rise to the throne of Ahmad's uncle, al-Mu'tamid. Real power however had come to lie with the elite Turkish troops and with Ahmad's own father, Talha, who, as the Caliphate's main military commander, served as the chief intermediary between the caliphal government and the Turks. Assuming the honorific name al-Muwaffaq in the style of the caliphs, Talha soon became the effective ruler of the Caliphate, a position consolidated in 882 after a failed attempt by al-Mu'tamid to flee to Egypt led to his confinement in house arrest.
Caliphal authority in the provinces collapsed during the "Anarchy at Samarra", with the result that by the 870s the central government had lost effective control over most of the Caliphate outside the metropolitan region of Iraq. In the west, Egypt had fallen under the control of Ahmad ibn Tulun, who also disputed control of Syria with al-Muwaffaq, while Khurasan and most of the Islamic East had been taken over by the Saffarids, who replaced the Abbasids' loyal client state, the Tahirids. Most of the Arabian peninsula was likewise lost to local potentates, while in Tabaristan a radical Zaydi Shi'a dynasty took power. Even in Iraq, the rebellion of the Zanj slaves threatened Baghdad itself, and further south the Qarmatians were a nascent threat. Al-Muwaffaq's regency was thus a continuous struggle to save the tottering Caliphate from collapse. His attempts to recover control of Egypt and Syria from Ibn Tulun failed, with the latter even able to expand his territory and obtain recognition as a hereditary ruler, but he succeeded in preserving the core of the Caliphate in Iraq by repelling a Saffarid invasion aimed at capturing Baghdad, and by subduing the Zanj after a long struggle.