He was born in Cairo in 603 AH.
He acted as his father’s deputy when al-Kāmil marched to besiege al-Nāṣir Dāwūd. On the Sultan’s return he took issue with several of his son’s actions and transferred his favour to another of his sons, al-ʿĀdil. Later, when al-Kāmil had captured Āmid, Ḥiṣn Kayfā, and Sinjār, he invested Najm al-Dīn with the sultanate and placed those territories under his authority. Najm al-Dīn remained there until he advanced and seized Damascus, after which he pressed on toward the Ghawr. Seizing the moment, his uncle Ismāʿīl pounced on Damascus and took it. The troops of al-Karak then descended, surrounded al-Ṣāliḥ, and carried him off to al-Karak. Al-Nāṣir escorted him away after the Kāmilid emirs had summoned him; they deposed his brother al-ʿĀdil, installed him on the throne, and al-Nāṣir returned empty-handed.
Ibn Wāṣil comments: “He never kept company with scholars and took no part in their pursuits, unlike his father.”
In 641 AH al-Ṣāliḥ and his uncle—who was likewise styled al-Ṣāliḥ (1)—concluded an agreement that Damascus should belong to the uncle, while he, together with the people of Aleppo and Ḥimṣ, would have the Friday sermon proclaimed in the name of al-Ṣāliḥ Najm al-Dīn. They also stipulated that his son al-Malik al-Mughīth, Ibn Abī ʿAlī, and Mujīr al-Dīn ibn Abī Zakarīyā be sent to him, which the uncle duly did.
The Ayyubid princes then united in enmity against the lord of al-Karak. Ismāʿīl dispatched an army to besiege ʿAjlūn, which was in al-Nāṣir’s hands, but the campaign disintegrated when Ismāʿīl found a letter from Ayyūb urging the Khwarazmians to come and besiege his uncle. He thereupon imprisoned al-Mughīth, made peace with the lord of al-Karak, allied himself with the rulers of Ḥimṣ and Aleppo, and sought support from the Franks. The Egyptians advanced under the command of Baybars al-Ṣāliḥī al-Bunduqdārī the Elder, the very man who later killed his master. Ismāʿīl, however, ceded Jerusalem to the Franks; they rebuilt Tiberias and Ascalon, monks set wine-jars upon the Rock, and the call to prayer was silenced.