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阿布·阿拔斯是先知穆罕默德的叔父阿拔斯·伊本·阿卜杜勒·穆塔里卜的后代。在阿拉伯人的第一个王朝倭马亚王朝统治的末期，阿布·阿拔斯加入主要争夺权力者的行列。他在生于呼罗珊的波斯释奴阿布·穆斯林领导的阿拔斯派武装力量帮助下反对倭马亚王朝的哈里发。什叶派人士因为仇恨倭马亚家族而把他视为救星，对他推翻倭马亚王朝的努力提供了最大的帮助。他也得到了哈瓦利吉派的支持，尽管这些人在阿拔斯王朝建立之后就不断发动叛乱以致被消灭。
阿布·阿拔斯在军事方面完全依仗他最杰出的将领阿布·穆斯林。747年阿布·穆斯林在莫夫绿洲发动的叛乱得到了伊朗人和部分阿拉伯部落的支持，打响了推翻倭马亚王朝的第一枪。750年1月，阿布·阿拔斯在大扎卜河战役中彻底粉碎了倭马亚王朝最后一任哈里发马尔万二世的军队，取得了决定性的胜利。此役相当于宣告了倭马亚王朝的灭亡。阿布·阿拔斯在掌握政权之后，把倭马亚家族成员几乎斩尽杀绝。在什叶派支持下，阿布·阿拔斯成为哈里发，他在就职演说中自称为“萨法赫”（屠夫）。他在掌权之后立刻任命阿布·穆斯林为呼罗珊总督。
754年，阿布·阿拔斯-萨法赫把帝国的都城由倭马亚王朝支持者众多的大马士革（位于叙利亚境内）迁至幼发拉底河畔的安巴尔（位于伊拉克境内）。
As-Saffah the head of one branch of the Banu Hashim, who traced their lineage to Hashim, a great-grandfather of Muhammad, via al-Abbas, an uncle of the prophet. The Banu Hashim had great support from the camp of Ali, the fourth caliph. They thought that the family which had produced Muhammad and Ali would produce another great leader or mahdi who would liberate Islam. The half-hearted policies of the late Umayyads to tolerate non-Arab Muslims and Shi'as had failed to quell unrest among these minorities.
This unrest led to revolt during the reign of Hisham ibn Abd al-Malik in Kufa, a prominent city in southern Iraq. Shi'ites revolted in 736 and held the city until 740, led by Zayd ibn Ali, a grandson of Husayn and another member of the Banu Hashim. Zayd's rebellion failed, and was put down by Umayyad armies in 740. The revolt in Kufa indicated both the strength of the Umayyads and the growing unrest in the Muslim world.
As-Saffah chose to focus on Khurasan, an important military region in eastern Iran. In 743, the death of the Umayyad Caliph Hisham provoked a civil war in the Islamic Empire. Abu al-`Abbas, supported by Shi'as, Kharijis, and the residents of Khurasan, led his forces to victory over the Umayyads and ultimately deposed the last Umayyad caliph, Marwan II, in 750. The civil war was marked by millennial prophecies encouraged by the beliefs of some Shi'as that as-Saffah was the mahdi. Prominent Islamic scholars wrote works such as the Jafr telling faithful Muslims that the brutal civil war was the great conflict between good and evil. The choice of the Umayyads to enter battle with white flags and the Abbasids to enter with black encouraged such theories. The color white, however, was regarded in much of Persia as a sign of mourning.
Concerned that there would be a return of Umayyad power, as-Saffah invited all of the remaining members of the Umayyad family to a dinner party where he had them clubbed to death before the first course, which was then served to the hosts. The only survivor, Abd ar-Rahman I escaped to al-Andalus (Spain), where the Umayyad caliphate would endure for three centuries. For his ruthless efforts to eliminate the Umayyad family, Abu al-`Abbas `Abdu’llah gained the epithet al-Saffah, which means the slaughterer or 'Shedder of Blood'.
After the victory over the Umayyads, Abu al-`Abbas's short reign was marked with efforts to consolidate and rebuild the Caliphate. His supporters were represented in the new government, but apart from his policy toward the Umayyad family, as-Saffah is widely viewed by historians as having been a mild victor. Jews, Nestorian Christians, and Persians were well-represented in Abu al-`Abbas's government and in succeeding Abbasid administrations. Education was also encouraged, and the first paper mills, staffed by skilled Chinese prisoners captured at the Battle of Talas, were set up in Samarkand.
Equally revolutionary was Abu al-`Abbas's reform of the army, which came to include non-Muslims and non-Arabs in sharp contrast to the Umayyads who refused any soldiers of either type. As-Saffah selected the gifted Abu Muslim as his military commander, an officer who would serve until 755 in the Abbasid army.
As-Saffah turned back on his promises to the Shi'a community in claiming the Caliphate for himself. The Shi'a had hoped that their imam would be named head of the Caliphate, inaugurating the era of peace and prosperity the millenialists had believed would come. The betrayal alienated Abu al-`Abbas's Shi'a supporters, although the continued amity of other groups made Abbasid rule markedly more solvent than Umayyad.
Abu al-`Abbas `Abdu’llah as-Saffah died of smallpox on June 10, 754, only four years after deposing the Umayyads. He appointed his brother Abu Ja'far al-Mansur and then Isa ibn Musa as his successors.