He inherited a newly independent kingdom from his elder brother Saw O but spent much of his reign putting down rebellions.
Although he regained the Mon-speaking provinces of Lower Burma, he could not recover the Tenasserim coast from Martaban's former overlord Sukhothai.
His whereabouts during the reign of his eldest brother King Saw O (r. 1311–1323) is unknown except that Zein was married to his first cousin Princess Sanda Min Hla, and had three children by 1323.
[2][6] However, in the tradition of the prevailing Southeast Asian administrative model, every new high king had to establish his authority with the vassals all over again.
The 19-year-old king garnered no initial support, and the vassal rulers in the Pegu province and delta promptly revolted.
With the renewed force, Zein then invaded the delta, and got the lords of Myaungmya and Bassein (Pathein) to submit.
The king of Sukhothai treated him like a vassal, giving him the title Binnya Ran De (ဗညား ရံဒယ်, Burmese pronunciation: [bəɲá jàɴ dɛ̀]).
[5] After the truce with Sukhothai, Zein tried to pick off Prome, then a nominal vassal of the northern Pinya Kingdom.
But the usurper was killed a week later in a putsch organized by Zein's chief queen Sanda Min Hla.