He and his brother Ahmad took the reins of power from their ailing brother Mulhim Shihab and ruled jointly until Mansur became the sole emir after winning a power struggle with Ahmad in 1763. Mansur aligned himself with Zahir al-Umar and Ali Bey, the autonomous rulers of Galilee and Egypt, respectively, in their rebellion against the Ottomans. Mansur was subsequently forced by the Druze sheikhs of Mount Lebanon to step down in favor his nephew Yusuf Shihab after Zahir and Ali Bey were defeated in 1770.