He was born in 302/914 in Kairwan, the first Fatimid Imam to be born in Maghrib. His name was Ismail and kunya was Abu Tahir, surnamed al-Mansur bi-Amrillah (Victorious by the command of God). He acceded the throne on 334/945 during the time when Abu Yazid had laid a seige over Susa.
It may be known that kunya is a part of the Arab personal name, being an appellation consisting of Abu (father of) or Umm (mother of) and followed by a name, usually that of the bearer's eldest son.