He was the son of sultan Mehmed I of Great Seljuq. At the age of twelve (1120–1121), he rebelled unsuccessfully against his elder brother, Mahmud II, who however forgave him. At Mahmud's death in 1131, the power was contended between Mahmud's son, Dawud, Mas'ud, whose powerbase was in Iraq, Seljuq-Shah (in Fars and Khuzistan) and Toghrul II. In 1133 Mas'ud was able to obtain recognition as sultan from the emirs of Baghdad, and to receive the investiture by caliph al-Mustarshid. Toghrul, who controlling the eastern provinces of the western Seljuq, launched a military campaign but was defeated by Mas'ud in May 1133. Toghrul died in 1134. Also in 1133 Mas'ud supported Zengi, besieged by al-Mustarshid's troops in Mosul.
In 1135 caliph al-Mustarshid contested his authority but, on 14 June of that year, he was defeated and made prisoner at Daimarg, between Hamadan and Baghdad, and killed two months later by the Hashshashins. As al-Mustarshid's successor, al-Rashid, also rebelled with the support of Zengi, Mas'ud besieged him in Baghdad, forcing him to flee to Mosul, where he was also killed by the Hashshashins. Although able to ensure control over Iraq, Mas'ud's power over the rest of the Suljuq empire was uncertain: apart from Khorasan and Transoxiana, which had been long time under the control of his uncle Ahmed Sanjar, Dawud kept control over Azerbaijan for several years, while weastern Persia was effectively ruled by emir Bozaba until Mas'ud defeated him, together with other emirs, in 1147. In 1148 Mas'ud faced another coalition against him, this time aiming to place Malik Shah on the throne in his place.
During his troublesome reign, Mas'ud was forced to accept to delegate his authority to numerous emirs with the Iqta', a tax institution which reduced the imperial incomes. Other became effectively independent sultans, such as Zengi. This caused, according to historian ibn al-Athir, the beginning of the steep decline of the Seljuq Empire.