mu'in ad-din was originally a mameluk in the army of toghtekin, the founder of the burid dynasty of damascus. when zengi, the atabeg of aleppo, besieged damascus in 1135, mu'in ad-din was at the head of army defending the city. that year the burid shihab ad-din mahmud took control of damascus after the assassination of his brother; when zengi gave up the siege and instead besieged homs, shihab ad-din sent yusuf ibn firuz and mu'in ad-din to govern it, with yusuf acting as mu'in ad-din's lieutenant. in 1137 mu'in ad-din was still governor of homs when the city was briefly besieged again by zengi. in 1138, shihab ad-din appointed mu'in ad-din atabeg of damascus and gave him the title "isfahsaller". later in 1138, zengi negotiated a marriage between himself and shihab ad-din's mother khatun safwat al-mulk, and as part of the settlement zengi received homs. mu'in ad-din was given the castle of barin in place of homs. on june 22, 1139, shihab ad-din was assassinated in damascus; jamal ad-din, emir of baalbek, was chosen as his successor, and mu'in ad-din was chosen to govern baalbek in his absence. he was in charge of the defenses of baalbek when zengi arrived to avenge the murder of his stepson. zengi besieged it with 14 catapults and the city surrendered to him.