He was the eldest son of Imad al-Din Zengi of Mosul, and the elder brother of Nur ad-Din.
In 1146 Imad al-Din Zengi was besieging the fortress of Qal'at Ja'bar when he was assassinated on September 15 by one of his servants who wanted to escape punishment.
[1] Two years before, the Seljuk sultan Mahmud II had named his cadet son Alp-Arslan as overlord of Zengi, but the latter had neutralized him and carried with him at the siege.
Two of Zengi's advisors, the head of the diwan al-Din Muhammad Jamal and hajib Amir Salah al-Din Muhammad al-Yaghsiyani took the side of Saif ad-Din: taking advantage of the inexperience of the young Seljuk, giving Saif ad-Din the time necessary to take control of Mosul.
The atabeg of the city, Mu'in ad-Din Unur, however refused them entrance, using the presence of Zangi's sons to convince the Crusaders to release the siege.