Patricia Crone describes Mansur as "a coarse soldier equally devoid of nobility and piety" who was "shunned by devout contemporaries" as he disregarded religion and was motivated solely by his desire to avenge the torture and murder of the Yaman champion, Khalid al-Qasri, by the ardently pro-Qays governor of Iraq, Yusuf ibn Umar al-Thaqafi, in 743.
[1][2] A member of the Amir branch of the Banu Kalb tribe, he began his career possibly in Iraq, where the tribe had settled, but appears for the first time in Syria as a member of the plot to overthrow Caliph al-Walid II in early 744.
[1] After Walid's murder, his successor Yazid III favoured the Yaman faction, and appointed Mansur as governor of Iraq in succession to Yusuf al-Thaqafi, perhaps as a deputy of al-Harith ibn al-Abbas ibn al-Walid.
He continued fighting alongside the Kharijites until Marwan II's general Yazid ibn Hubayra defeated them in 747.
Like many opponents of Marwan, he fled to Fars and joined the forces of the Alid rebel Abdallah ibn Mu'awiya.