Modern historians regard al-Mansur as the real founder of the Abbasid Caliphate, one of the largest polities in world history, for his role in stabilizing and institutionalizing the dynasty.

[7] Al-Mansur's brother al-Saffah began asserting his claim to become caliph in the 740s and became particularly active in Khorasan, an area where non-Arab Muslims lived.

Al-Mansur fled with the rest of his family to Kufa where some of the Khorasanian rebel leaders gave their allegiance to his brother al-Saffah.

During his brother's reign, al-Mansur led an army to Mesopotamia where he received a submission from the governor after informing him of the last Umayyad Caliph's death.

[8]: 24 Mansur's first wife was a Yemeni woman from a royal family; his second was a descendant of a hero of the early Muslim conquests; his third was an Iranian servant.

This agreement was supposed to resolve rivalries in the Abbasid family, but al-Mansur's right to accession was particularly challenged by his uncle Abdullah ibn Ali.

[12] Fearing the increasing power of the Abbasid general Abu Muslim, who gained popularity among the people, al-Mansur carefully planned his assassination.

[13] John Aikin, in his work General Biography, narrates that Mansur, not content with the assassination, committed "outrages on the dead body, and kept it several days in order to glut his eyes with the spectacle.

In 755 Sunpadh, an Iranian nobleman from the House of Karen, led a revolt against al-Mansur, taking the cities of Nishapur, Qumis, and Ray.

[13]: 201  Al-Mansur ordered a force of 10,000 under Abbasid commander Jahwar ibn Marrar al-lijli to march without delay to Khorasan to fight the rebellion.

Angered by al-Mansur's avarice, general Jahwar gained support from his troops for his plans to split the treasures evenly, and revolted against the caliph.

[13] In 759, al-Mansur sent an army under his generals Abu al-Khaṣīb Marzuq and Khazim ibn Khuzayma to Tabaristan to punish Khurshid for his support to Sunpadh.

In this same year, he confronted a group of the Rawandiyya from the region of Greater Khorasan that were performing circumambulation around his palace as an act of worship.

[15][11]: 201  When in 758/9 the people of Khorasan rioted against al-Mansur in the battle of Al Hashimiya, Ma'n ibn Za'ida al-Shaybani, a general from the Shayban tribe and companion of Yazid ibn Umar al-Fazari, the Umayyad governor of Iraq, appeared at the scene of the uprising completely masked, and threw himself between the crowd and Mansur, driving the insurgents away.

[17] Al-Mansur laid the foundations of Baghdad near the old capital of al-Mada'in, on the western bank of the Tigris River, a location acceptable to him and his commanders.

The circular city of about 2.4 km diameter was enclosed by a double-thick defensive wall with four gates named Kufa, Syria, Khorasan, and Basra.

[18] Al-Mansur had built Baghdad in response to a growing concern from the chief towns in Iraq, Basra, and Kufa that there was lack of solidity within the regime after the death of Abu'l 'Abbas (later known as al-Saffah).

Another reason for the construction of the new capital was the growing need to house and provide stability for a rapidly developing Abbasid bureaucracy forged under the influence of Iranian ideals.

[3] The medieval historians al-Tabari and al-Khatib al-Baghdadi would later claim that al-Mansur had ordered the demolition of the Khosrow palace in Ctesiphon so that the material could be used for the construction of the city of peace.

"[23] Al-Mansur's secret service extended to remote regions of his empire, and were cognizant of everything from social unrest to the price of figs, making Mansur very knowledgeable of his domains.

According to historic sources al-Mansur advised his son: “put not off the work of today until tomorrow and attend in person to the affairs of state.

For when sleep fell upon his eyes, his spirit remained awake.” Notably frugal, al-Mansur was nicknamed Abu al-Duwaneek (“the Father of Small Change”), kept close tabs on his tax collectors, and made sure public spending was carefully monitored.

While al-Mansur's regime did not intrude into the private realm of elites, orthodoxy was promoted in public worship, for example through the organization of pilgrim caravans.

[3] Imam Ja'far al-Sadiq became the victim of harassment by the Abbasid family and in response to his growing popularity among the people was eventually poisoned on the orders of the caliph[26] in the tenth year of al-Mansur's reign.

[30] Al-Mansur had Persian books on astronomy, mathematics, medicine, philosophy and other sciences translated in a systematic campaign to collect knowledge.

The translation and study of works in Pahlavi, a pre-Islamic language of Persia, became fashionable among intellectuals and authors who supported the Abbasid caliphate.

[34] In 765 al-Mansur suffered from a stomach ailment and called the Christian Syriac-speaking physician Jurjis ibn Bukhtishu from Gundeshapur to Baghdad for medical treatment.

In fact, it was the first Carolingian king Pippin III who initiated a new era of Franconian diplomacy by sending diplomatic envoys to al-Mansur's Baghdad court in 765.

One account narrates that al-Mansur was on a pilgrimage to Mecca and had nearly reached, when death overtook him at a location called the Garden of the Bani Amir on the high road to Iraq at the age of sixty-three.

[49] A different narration from Fadl ibn Rabi'ah, who claimed to have been with Mansur at his time of death, states that he died at al-Batha' near the Well of Maimun in which he would have been buried at al-Hajun at sixty-five years of age.