In 1221 as-Salih became a hostage at the end of the Fifth Crusade, while John of Brienne became a hostage of as-Salih's father Al-Kamil, until Damietta was reconstructed and restored to Egypt. In 1232 he was given Hasankeyf in the Jazirah (now part of Turkey), which his father had captured from the Artuqids. In 1234 his father sent him to rule Damascus, removing him from the succession in Egypt after suspecting him of conspiring against him with the Mamluks. His uncle as-Salih Ismail soon expelled him from Damascus, and he fled to the Jazirah, where he allied with the Khwarezmians.
In 1238 al-Kamil died leaving as-Salih his designated heir in the Jazira, and his other son Al-Adil II as his heir in Egypt. In the dynastic disputes which followed, as-Salih took control of Damascus, in 1239, and set about using it as a base for enlarging his domain. He received representations from his father's old Emirs in Egypt, who appealed to him to remove his brother, and in early 1240, while making ready to invade Egypt, he was informed that his brother had been captured by his soldiers and was being held prisoner. As-Salih was invited to come at once and assume the Sultanate. In June 1240, As-Salih made a triumphal entry into Cairo and became paramount ruler of the Ayyubid family.