A veteran of campaigns under Caliph al-Mu'tadid, he distinguished himself by saving the young Caliph al-Muqtadir from a palace coup in 908. With the Caliph's support, he became commander-in-chief of the caliphal army, in which role he served in several expeditions against the Byzantine Empire, saved Baghdad from the Qarmatians in 927 and defeated two Fatimid invasions of Egypt, in 915 and 920. In 924 he helped secure the dismissal and execution of the vizier Ibn al-Furat, after which his political influence grew enormously, to the point that he briefly deposed al-Muqtadir in 928. His rivalry with the Caliph and with the civilian bureaucracy of the court finally resulted in an open confrontation in 931–932, that ended with Mu'nis's victory and the Caliph's death in battle. Mu'nis installed a new caliph, al-Qahir, but in August 933 the latter had Mu'nis and his senior officers executed. Mu'nis's usurpation of power, just as his violent end, marked the beginning of a new period of turmoil for the declining Abbasid Caliphate, culminating in its takeover by the Buyids in 946.
According to the 14th-century account of al-Dhahabi, Mu'nis was 90 years old at his death, indicating a birth ca. 845/6. He was a eunuch slave, and is hence called al-Khadim ("the Eunuch") in the sources to distinguish him from his contemporary colleague, the treasurer Mu'nis al-Fahl ("the Stallion"). He first appears as a ghulam of the future caliph al-Mu'tadid (reigned 892–902) during the suppression of the Zanj Rebellion in 880/1, and had risen to the position of chief of police (sahib al-shurta) in al-Mu'tadid's camp by 900. Al-Dhahabi, however, records that the caliph banished him to Mecca, whence he was recalled only after the accession of al-Muqtadir (r. 908–932) in 908, a statement apparently corroborated by his complete absence from the sources during the intervening reign of al-Muktafi.
Mu'nis rose to prominence early during the reign of al-Muqtadir: in 908, shortly after the Caliph's accession, a faction of the bureaucracy and the army launched a coup to depose him and replace him with his brother Abdallah ibn al-Mu'tazz. Mu'nis led the defence of the palace and the coup collapsed. This earned him the gratitude and support of the young caliph and his influential mother, Shaghab, and solidified his position among the grandees of the Abbasid court. Mu'nis became the commander-in-chief of the Abbasid standing army, a force numbering 9,000 men in 927. In 909 he led the customary summer raid (sa’ifa) against the Byzantine Empire, launching an invasion of Byzantine Asia Minor from Malatya and returning with many prisoners. In the next year, he succeeded in recovering the province of Fars from the declining Saffarids, taking advantage of the strife between the Saffarid emir al-Layth and the former Saffarid general Sebük-eri, who had seized control of the province. When al-Layth's brother al-Mu'addal invaded Fars, Sebük-eri called on the caliph for aid, and an army under Mu'nis was sent. Al-Layth was defeated and captured, while Sebük-eri was soon deposed as governor when he failed to gather the promised tribute. In the same year, 909/10, Mu'nis supervised a prisoner exchange with the Byzantines.
In 914, the Fatimids, who had only a few years before taken over Ifriqiya by ousting the reigning Aghlabids, launched an invasion of Egypt under Abu'l-Qasim, the future caliph al-Qa'im bi-Amr Allah. The Fatimids succeeded in capturing Alexandria, but failed to capture the province's capital at Fustat. In 915, Mu'nis led Abbasid reinforcements to Egypt and drove them out of the country again, for which he earned the honorific laqab of al-Muzaffar. On his return from Egypt, he was ordered to proceed to the Jaziran border zone (thughur), where the Byzantines, taking advantage of the rebellion of Husayn ibn Hamdan, had captured the fortress of Hisn Mansur and deported its population. In retaliation, he led a major raid in late summer 916, capturing several fortresses in the vicinity of Malatya, while ordering Abu'l-Qasim Ali to lead another raid from Tarsus. In September/October 917, in response to a Byzantine embassy led by John Rhadenos, he supervised, along with Bishr al-Afshini, the governor of Tarsus and the Cilician border zone, another prisoner exchange on the Lamos River.
In 918–919, Mu'nis campaigned against the rebellious ruler of Adharbayjan, the Sajid Yusuf ibn Abi'l-Saj, who withheld part of the taxes owed to Baghdad and had even seized provinces in northern Iran from the Samanids without the Caliph's approval. In his first campaign in 918, Yusuf initially withdrew before Mu'nis to his capital, Ardabil. After attempts at mediation with the Caliph by the vizier Ibn al-Furat failed, Yusuf confronted Mu'nis in a pitched battle before Ardabil, where Mu'nis was defeated. In the next year, however, Mu'nis defeated Yusuf in a second battle before Ardabil and took him as a prisoner to Baghdad. Yusuf remained captive in Baghdad for three years, while in the meantime, Yusuf's ghulam Subuk held power in Adharbayjan, having secured the Caliph's recognition. It was Mu'nis who was responsible for persuading al-Muqtadir to release Yusuf in 922 and restore him to his old position, this time as a servant of the Abbasid government. In 920–922, Mu'nis was instrumental in defeating a second Fatimid army sent to take Egypt. The Fatimids once again took Alexandria and occupied the Fayyum, but their fleet was sunk and Mu'nis defeated their army before Fustat, trapping Abu'l-Qasim in the Fayyum, from which he was able to escape only with heavy losses. In 923, he launched another raid into Byzantine territory, capturing a few forts and returning with much booty.